Merged mining is quite interesting, but how about the efficiency? If I have 1 M of scrypt-N hash power, and I put it into a merged pool like VTC/PLX pair, do I have 1 M of hash power in both coins?
The hashing is equal on both coins

you would retain your 1 M of hash power on both coins.
There is what looks good anymore this coin?
Can be minning together only?
If I understand correctly, no, you can also exclusively mine PLX. Check out the OP (first post of this thread) for the
non-merged pools, there you can mine for just PLX.