[Suicide Junkie]

That is the way OR works. If at least one side is true, then the result of ORing them is true.

The simplest way to make it exclusive would be to add a requirement of (count(A)=0 OR count(B)=0)

If you want to handle more than just two types, then try checking to ensure that the maximum of the counts (IE the type with the most components installed) is equal to the sum of the number of engines installed of all types.
Eg: max(a, max(b, max(c,d))) = (a+b+c+d)