Not the same -- the 1.5" pumps significantly more. Think about it this way. If you calculate the area of a circle with a 3/4" diameter you get *way* less than half of the area of a circle with a 1.5" diameter.
That said, if you have two 3/4" pipes to work with, that's certainly better than one!! It's not ideal, but I think it's workable. However, I'm a big fan of the herbie system, and in that you leave one of the drainpipes completely open as the emergency drain. I'm not sure if it would be wise to go that route with your two pipes, but that might be preferable to using both to drain. Hopefully someone with more experience can chime in on that issue.