By adding a second set of wires to that meter base, you are creating a second 'main' service. The NEC generally frowns on this. I've encountered this sort of thing only rarely, usually in some sort of duplex or apartment conversion, or in a farm setting (where one meter serves several buildings).
Interesting. That is a somewhat common installation here, using a feed thru Meter base with dual load lugs. It was more popular before the all in ones took over though, now we just use a 400 all in one with a second 200 amp sub feed breaker. I have had a few service replacement situations where that wouldn't work and we went with the dual feed meter base and fed into two main breaker panels.