Uihlein, who shared an apartment with compatriot Koepka in his first few years as a professional, carded a second consecutive 67 at Le Golf National to join Spain's Adrian Otaegui at the top of the leaderboard on eight under par.
Otaegui had set the early clubhouse target following a superb 66, before an afternoon thunderstorm led to a delay of two hours and five minutes. After going to the turn in 34, a hat-trick of birdies from the 13th took Uihlein into the outright lead, only for the 27-year-old to card his only bogey of the first two days on the 17th.
"It's always quite tricky when you have to go back out," the 26-year-old from Southport said. "It was a shame because I felt like I had a lovely rhythm again today. You don't want to come in, but it was fine. At least we got to finish.
"It's nice to finish this evening rather than getting up early tomorrow morning and coming back. The last two holes are tough and a couple of pars there is a nice end to the day really."
First-round leader Paul Waring could only add a 72 to his opening 64 to finish six under par alongside compatriot Nathan Kimsey, with Graeme Storm, Ross Fisher and Ryder Cup star Thomas Pieters a shot further back.
"I putted well, that's the only positive out of today," Pieters told Sky Sports after a 71 featuring four birdies, two bogeys and a double bogey. "I scrambled well on some holes, but my putting has definitely been very good the last two days.
"I didn't hit it well off the tee today and it cost me four or five shots; it's definitely a second-shot golf course. If you can place it in the fairway you don't need to be very long. If you're hitting from the short grass it's a big advantage, especially with the rough being wet. I've got some work to do but I won't be calling my coach Pete Cowen. I know what I have to do."
Rising Spanish star Jon Rahm was also set to spend some time on the range, despite adding a 69 to his opening 70 to finish three under.
"My game off the tee, I wish it could be better," the 22-year-old world No. 11 said. "I've been trying to fix it. I don't know what's going on. At least now I'm missing right instead of left which is something I can play with.
"But I've been lucky to manage myself around and shoot under par twice, with only five fairways today and five yesterday, which is something that's pretty unusual here.
"My ball-striking after the tee shot feels great. My irons look good. My short game feels great. My putting feels great. I've been making a lot of long putts. I just need to give myself a chance to be on the fairway and maybe make some birdies."
Uihlein, who missed the cut at Erin Hills while Koepka powered to a record-equalling victory, said: "I've holed a decent amount of putts, which I didn't do at all at the US Open. To come out a week and a half later and start holing some putts takes a lot of pressure off a lot of things. Makes it a little more comforting.
"We still have 36 holes left, so you never know. But I think any time you can put yourself into contention after two days is where you want to be and see where you're at as the weekend plays out."