You cannot stack 25%, 40% or 100% Bonus EXP and GP Passes together to increase their boost amount. You may only have one EXP and one GP pass equipped at a time.
Only EXP+GP Combo Passes can be stacked with other passes. The Combo Pass can be equipped alongside both EXP and GP Bonus Passes (example: 100% EXP Pass, 40% GP Pass, 20% Combo Pass).
This rare type of Bonus Pass cannot normally be bought from the shop or received through supply cases, so stock up on it when you see it offered!