Originally Posted by dabac

Thing is, only because you're pushing with equal force on both sides, there's no guarantee that they will move equally.

I used a modified Sheldon, laid the bike on a workbench with the rear triangle hanging over the edge, stuck a 2x2 under the dropout, leveraged it against the seat tube and pushed a bit. When it had moved out half the intended spread I flipped the bike over and did the other side. Minimal drama.