To clarify, you must be one size smaller than a creature to 'ride' it. If you are not, and it still has enough strength to pick you up, it may 'carry' you, but you don't count as 'riding' it.

Trying to apply the bonuses of a single-target magic item like a belt to two creatures at the same time has nothing to do with bending the rules or creative interpretation or anything else - you are straight up cheating.