It is common practice around here, when adding an off-peak meter to simply nipple a second meter socket to the original one and tap onto the line side lugs to feed the second socket.
What to do?

Redsy,
A 2 ganger would be the right way,
besides you did'nt do it, so no sweat right??

I've done it both ways. The local PoCo. shows this method in their guide book.