A 501© 3 has come to us and are maintaining both a 401k and 403b. It appears there are a few employees deferring to both plans in same year. I thought you could not sponsor both plans but had to choose one or the other.

An employee may defer to both, but the two are aggregated for 402(g) purposes. (In other words, total deferrals between the two can't exceed the 15,000 plus catch up if applicable.)

Don't get 402(g) confused with 415 here - 415 can get a little trickier.