You can try to bake both at the lower temperature, and it might turn out OK, but there are a lot of variables that could cause it not to.
I went into some of the effects of time and temperature in this answer. In a nutshell, you have two major processes happening when you bake dough; the first is the Maillard reaction (browning) and the second is water evaporation (drying/hardening). Just to make things even more fun, the second has a strong effect on the first; the more water evaporates, the quicker the browning will occur.
There's also the gluten development (chewiness) of the crust, but that is less of an issue with pizza because you're usually aiming for a crispy crust.
What this means to you is that if the two different pizzas have crusts with different amounts of water, then the one with lower water content may end up rock-hard and even burnt by the time the second one is properly cooked.
350° F is only a little bit higher than Maillard temperature, so the browning will happen pretty slowly; the fact that the box suggests this temperature likely means a relatively high water content, so that the crust can firm up before starting to brown. On the other hand, 425° F and 20-minute cooking time probably indicates a higher water content; you want to brown the crust more quickly before the crust gets too hard and/or starts to burn.
So there's a non-trivial likelihood that after 33 minutes at 350° F, the second pizza (which gives a higher temperature) will be undercooked and doughy. Undercooked is easy to fix in theory, but the problem is that you've thrown off the water ratio, which was specifically formulated for a higher temperature, so it's going to be hard for you to get it exactly right, even if you raise the temperature to finish it off. It'll still be edible, of course; it's really just a matter of how picky you are (the answer to which is probably "not very" if you're eating frozen pizza...).
On the other hand, there's also a not-too-distant possibility that the two pizza crusts are practically the same, and the different brands just have different ideas of how crisp/brown the crust ought to be when cooked. It's really hard to know until you try.
One other thing to mention is that if you plan to bake both pizzas at the same time then you will probably need to raise the temperature/time slightly to compensate. Also, I really wouldn't recommend putting one on a lower rack, because it blocks the heat from the element and tends to mess up what little circulation there is in a conventional oven, and you'll end up with a very flat and probably undercooked pizza.
Bakeries and pizzerias will usually have pizza stones and special ovens with fans and possibly turntables to mitigate this issue; home ovens don't. So I'd recommend either baking them on the same rack (if you have room) or baking them separately.
I would cook the Brie at that same temperature for 30-35 minutes and it should be okay.
Alternatively, you could cook the roast fully, remove it from the oven, tent it with foil and then cook the Brie. The meat can rest while the Brie is cooking and being eaten. Prior to slicing the roast, heat up the pan drippings to a sizzling temp and baste to ensure a crispy crust. But if the appetizer course happens to last for 20-30 minutes, then the roast will have been resting for 40-50 minutes and could lose too much internal heat.
Best Answer
Yep, you can cook them together. Compared to others this time of year, you've got it easy. Keep an eye on them, sometimes one dish can block the other from cooking evenly, especially if they are on different shelves. You may need to rotate, so give yourself a little extra time as that will cause the oven to cool down, as will adding your second dish after the first one has started.