If you sign a Mutual Defense Pact with a realm, but you are just a peace with them, you won't be able to do much to defend them anyway. Only one of you would participate in battles anyway, the other would just observe.
Just to clarify the game mechanics: non-allies can participate on the same side in battle if they are attacking someone they are both at war with. They won't fight together if they would be the defender in the battle.
You can see more information on how sides are determined on the wiki:
https://wiki.battlemaster.org/wiki/Battle/Diplomacy_Effects
