AFAIK there is no rule forbidding somebody to keep 2 GCs at a time. In fact, many do keep two cards due to conditional card and permanent GC.
I removed my conditions on GC, and still have expired GC + extension letter extending its validity.
OP can return GC during naturalization or next GC renewal, but I am not aware of requirements to return second GC.
As long as OP doesn't lend it, sell it or misuse it, it's just another proof of status.