Double ovulation would dramatically increase the possibility of twins. However, I don't think it is very common for both follicles in a double to be the exact same size, unless one came from each ovary. (You also mean to say the follicles are 32mm not cm.)
I don't know if it technically increases your chances of conception. If they both ovulate at the same time, I would say the only increase is that you have two available for fertilization. If they ovulate at different times, the increase would be that you have a larger window of time for at least one to be fertilized. That issue is something you would have to try to research.
I also suggest this article Predicting Ovulation in the Mare
(Because who doesn't love Equine-Repro!!)
If I were you though, I would be calling the stallion owner, because I bet you will need semen tomorrow, and if they shipped it tomorrow it will probably be too late.