I want it to give double of what the original game does per move with out JP UP equipped . Oh and for the both versions . PSP PSX
The slightly tedious and non ASM way is to give everyone Gained JP Up innately. This assumes you're getting rid of it as a regular support ability, which is probably a good idea anyway.
If you want to keep Gained JP Up as a support ability and double JP gained, that'd require a bit of asm, which I can't personally provide.
Or you could just lower the amount of JP needed to go up job levels and learn skills across the board for a similar effect. I assume you are looking to do this to lower the amount of time you need to grind.