The second fatality was identified as a woman who was killed as she drove through flooded streets on Houston's west side, a Houston police officer said on Saturday.
On Friday night, an unidentified victim died in a house fire in the devastated coastal town of Rockport.
Another dozen people in the area suffered injuries like broken bones, another official said.
Harvey slammed into Texas late on Friday as a Category 4 hurricane with winds of 209km/h, making it the strongest storm to hit the state since 1961. It has since lingered over Houston, dumping torrential rain in the city.
The powerful storm left a trail of destruction after slamming into Texas, with officials warning of "catastrophic" flooding.
Rescue workers search for survivors in a structure damaged by Hurricane Harvey in Rockport, Texas, 26 August 2017. Source: AAP
After tens of thousands of people fled its impending onslaught, Harvey flattened buildings and severed power across a swathe of the Gulf Coast that is home to some of the country's most important oil refineries.
Officials say the worst may yet be to come, with the real danger coming from rains likely to inundate Texas for several days.
Harvey made landfall late Friday as a Category Four hurricane, pummelling the small town of Rockport outside Corpus Christi with sustained winds of 130 miles (215 kilometres) per hour.
It then made a second landfall a few hours later just north of Rockport as a Category Three hurricane, with winds of 125 mph, the National Hurricane Center (NHC) said.
While most residents did heed advice to head to safety, some hunkered down for the night in Corpus Christi - a city of about 325,000 - in buildings that seemed to bend in the wind.
"I've never seen anything like this. We do have strong winds - we're right next to the bay - but nothing like last night," store owner Brandon Gonzalez told AFP.
"I mean, I was even a little bit terrified of what was going to happen. Our building was just shaking back and forth. It really got bad. I think we held up pretty good though."
In the early hours of Saturday, Harvey lost strength as it moved inland and was downgraded to tropical storm status at midday.
But meteorologists said a vast area would face potentially devastating floods over the coming days, while some communities were hit by localised tornados.
Some areas could get up to 40 inches (more than 100 centimetres), according to the NHC, which warned of "extremely serious flooding."
A damaged car sits outside a heavily damaged apartment complex in Rockport, Texas, in the wake of Hurricane Harvey. Source: AAP
"Rainfall of this magnitude will cause catastrophic and life-threatening flooding," it added.
Although Texas Governor Greg Abbott told a press conference there were no confirmed casualties thus far, the Federal Emergency Management Agency said there should be no illusions about the long-term impact.
"This is going to be an unprecedented long and frustrating event for the state of Texas," FEMA director Brock Long told MSNBC.
"The recovery from this disaster is going to be years."
'Severe blow'
Emergency services were struggling to make headway as rains continued to lash down while the coast guard was responding to May Day calls from tugboats out at sea.
President Donald Trump, aware of the damage to George W. Bush presidency's for his tardy response to Hurricane Katrina, said he was closely monitoring relief efforts and urged everyone to "Be Safe!"
Trump and Vice President Mike Pence met with their cabinet on Saturday via teleconference to discuss the ongoing response to the mega-storm, the White House said.
Rockport Mayor CJ Wax said countless businesses and homes had been completely destroyed or suffered catastrophic damage.
"We have already taken a severe blow from the storm but we are anticipating another one when the flooding comes down from the interior of the state, as the storm stalls and then all the rain heads our way," he told CNN.
Rockport's local school and airport were among the places to suffer major damage while homes were also burnt to the ground as power cables caught fire. There were similar scenes in nearby Corpus Christi.
While the damage in Houston was less extensive, the suburb of Sienna Plantation suffered an apparent tornado hit which knocked down walls and tore tiles off roofs, according to the local KTRK network.
Abbott said his primary concern was of "dramatic flooding" and warned residents to be on guard for rapidly rising water.
So far, there have been 338,000 power outages across the state, he said.
Catastrophic flooding
Brian McNoldy, a hurricane expert at the University of Miami, said a strong ridge of high pressure was preventing the storm from dispersing.
"You would expect the hurricane to just keep going inland over Texas, but that ridge coming into place there is just stopping everything from moving and that's what's going to keep Harvey in place for the next days," he told AFP.
"A tropical cyclone - it could even be a depression, it doesn't matter - makes a lot of rain, so if that sits over you for one day, two or three or four, you just keep accumulating rain. It may stay three or four days, and even up to six days. There's no sign of it really moving in a foreseeable future."
Coastal Texas is a fast-growing area, with some 1.5 million people moving into the region since 1999. It is also home to a large number of oil refineries.
US authorities said about 22 percent of crude production in the Gulf of Mexico, accounting for more than 375,000 barrels a day, was shut down as of Friday.
Leaving everything
Harvey is the most powerful hurricane to hit the mainland since Wilma struck Florida 12 years ago.
2005 was a huge year for hurricanes - before Wilma, Hurricane Katrina pummeled New Orleans, leaving more than 1,800 dead.
Then-president Bush faced severe criticism after federal authorities appeared unprepared for the devastating damage.
Wary of any accusation of complacency, Trump said he was "closely monitoring" Harvey from his Camp David weekend retreat and granted Abbott's request to declare a "major disaster" zone to speed federal aid for victims.
Many residents who fled the worst-affected areas headed for the city of San Antonio, where temporary shelters are run by the fire department.
"I felt like I didn't want what happened to the guys in New Orleans... I didn't want that to happen to me," Michael Allen, an evacuee from Corpus Christi, told AFP.
"I only got what you see me with. Everything I had, I had to leave. Everything."
