Decking - Shorter Sister Joists for Abutted Boards?
As the sister joist is not to reinforce the structure of the deck, but only to support the ends of the floor planks then making if from multiple short pieces will not be a problem.

it doesn't even need to be the full size of the other joists, you can use 2x4 or whatever. is convenient.

It's also legal to rest two plank ends on a single joist (each end about half way across) but if your deck is too long for your planks to be used that way you can't do that.

I'd consider staggering the boards so that all the joins don't happen in a straight line.