Originally Posted by Duster_360

1. Yes, as long as the cell counts are the same, capacity can be diff. The C rating can be a prob if these are diff brands. There is no standard for establishing C rating and they can be diff from brand to brand. Keep an eye on temps when they're discharging, if they're truly the same C ratings, their temps should be the same. Also if there's a large age diff (number of cycles) between them, that can also cause probs.

2. They will discharge at same voltage - effectively what you're doing is creating a 9000mah 5S lipo. It will drain as such. If both are truly same C rating, they should discharge at same voltage. Again, if there's an age diff they may act differently.