Is it possible to place components on both sides of the board if I'm doing DIY toaster-oven style reflow soldering? I would think that doing the reflow twice (for the other side) might overheat the components already soldered on.
I have seen this done with a two step method.
The most professional way I have been taken part in involved all of the components have a bit of glue sticking them down(technically epoxy). The oven then would heat the top side to the correct temp, then the board was flipped and the board had the top heated. This was being used by a firm that created products exclusively for the military and I believe had strict requirements on reliability and I believe their oven method was very reliable.
I have also seen methods where people use high temp epoxy and then just bake the whole thing at once. You need things lined up carefully because the epoxy will stop things from moving around and normally during reflow parts move into their proper position on the board almost automatically.
I've found that most components will be held in place just by the surface tension of the liquid solder, when run through the re-flow oven inverted. You can get about 30g per square inch of pad area before the component will actually drop. If you do have a larger component on one side you run that side second. You only have to brace components if you have large components on both sides.
Use Kapton tape or a Teflon tape wrapped around a larger component to thermally shield it and prevent re-flow.
Make Jigs out of scrap FR-4 from the edges of your panelized boards. Components that won't be held by surface tension are usually taller than most of the components on a board. This means you might be able to jig it so that the boards weight rests on your heavy component pinning it in place.
For home DIY one of possible hacks is to use pastes with different liqid points. Reflow one side with 180C paste, then another side with 130C paste.
Also, you can rely on surface tension, if have no heavy components or place those on last reflow side
I would think that doing the reflow twice (for the other side) might overheat the components already soldered on.
For DIY that's usually not a problem. Of cause, everything depends on components & situation. Don't apply this answer "for professional use".