.. You top during veg stage...then when you flower you have 2+ tops...
I go to the second set of nodes (offshoots from stem)..cut straight across, so you don't expose to much of inner stem, about 1/4" up from new leaf growth...
Yeah you still can top it, but your not going to get the same kinda results as if you topped two weeks ago. You will probably end up with more a V shaped cola instead of two seperate main colas.