There is an FA shuttle that operates between the two at set times. But you could be sitting around for hours trying to get on it. Public transit is just under 2 hours, IIRC and an Uber runs $70-100.

The commute between the two should be pretty short lived though. As a lineholder, you’ll pick one or the other. For me, LGA is a much easier commute with lower load factors. It also depends what fleet. Bus does or did more LGA stuff.