Did you try this and what happened?
My case is a little different. I have AA awards ticketed on JAL and want to book the same flight and passengers in a different cabin. It's not a different routing. I will use AS miles. I'd rather not cancel the 1st until the 2nd are ticketed. JAL takes time to finalize an award, and I'm concerned they'll cancel one automatically while waiting. I'm trying to decide on the best strategy.
The JAL website
vaguely mentions they block duplicates at purchase time, but I assume they're referring to their website only.
Q. If you try to make a duplicate reservation, you will get an error.
A. In order for more customers to use their seats effectively, we may not be able to accept multiple reservations that are not actually available due to the schedule.