Cameron tested this. There was room for both on the door, but only in a very specific orientation could they both stay dry. And because she was wearing more clothes than he was, even if Jack makes it out of the water, he’s still hypothermic and won’t survive.
Edit: It’s a NatGeo doc called “Titanic: 25 years later.” They’re in a pool that’s 56 degrees F. You can find it on streaming someplace, but here’s a clip. https://www.youtube.com/watch?v=YEJph0aIP-U
Yeah, I haven’t actually sat down to watch the 1997 Titanic in a long time, for that reason. A Night to Remember exists. 1997 Titanic breaks in half, but in the wrong place, so they’re both wrong about it ;)