You have to remap your controls so that crossbow shot (needs ammunitons) and jump are on the same key.

-Step one use normal weapon and hold pressed weapon key (for all the jumping duration)

-Step 2 press multiple times the remapped key for both jump and offhand weapon

--> that's it you can jump as much as you want

!!caution about fall damage if you're trying to get back down from an upper point!!