There's this town with two blacksmiths - one, a swordsmaker and the other, a shield maker. The swordsmaker's swords can slice through anything as a rule and the other guy's shields cannot be destroyed. Now they get cut up with each other for some reason and pit their wares against each other. What do you expect happens?
This reminds me of the unmovable object(say a wall) vs the the object which can't be stopped(say a canonball).
If both of these were true, then the wall would have an infinite amount of inertia causing it to forever stand still and the canonball would have an infinite amount of inertia and would never stop or slow down. If both of these were the case then the canonball would continue pushing into the wall forever and the wall will never yield.
This gives into the paradox or impossible fact that two items can simultaneously have infinite inertia, ie if the wall has infinite inertia then there is none for the canonball.