Testing with the wipeout and loadouts of 10x gbu12 x2 pylons and 10x gbu12 pylons with priority of all pylons set to one, it is not possible to release more than one gbu12 at a time.
This is contrary to trying the same with the shriekers, having 10x shrieker HE x7 mounted, all on the same priority I could fire all pylons at once,
and also contrary to the description on the vehicle loadouts page:

can be used to make weapon fire in different order or to force burst of all weapons of same type