It will depend on your chainline. Some have a tendency to do what you are seeing and some are more tolerant. Do you have a triple crank so the crank is spaced further out from the centerline?

I have a Campy 10-speed triple and a Shimano 9-speed triple equipped bikes and neither will tolerate back pedaling in the big-big combination without spilling the chain off of the big cog.

On the other hand, I also have a Shimano 7-speed, double chainring bike and it will allow backpedalling in the big-big with no problem.