Originally Posted by
Andrew R Stewart
I pretty much agree with a comment or three.
The OP seems to be assuming that both stays will bend symmetrically. Often they don't because of reasons like drive side chain stays can have different indents for chain ring clearance that the LH side doesn't. This is the main reason why I do my rear triangle spreading one side at a time. I block the BB shell (what, you mean I have to do more work just to realign a frame...) in a bench vise and work on one set of stays, then the other. Besides controlling where you end more easily you also do a better job at maintaining the frame's chain line and tracking.
That is pretty much what I do too, but I also check alignment with a frame alignment gage between each movement, as well as where the dropout spacing is at. I have a ***-2 Park tool but the string method works fine. This helps me tell which side needs to move and helps prevent going one way too far causing a frame alignment issue. In most cases I find the drive side moves easier as you have mentioned.