Convenient? not so much. Possible. Yes.
-One could rent a car between the two.
-There are direct shuttle services that operate between the two by private companies.
-A car service can be hired if the shuttle service doesn't work for you.
-A train can be taken between from JFK's Jamaica Station (required free AirTrain ride first) and then changing in Penn Stations to a NJ Transit train for a ride to EWR station where another Air Train ride is needed. A similar option is available by using two Airport Buses routed via Manhattan. Upsides to this is higher frequency than the direct shuttle service & pickup/drop-off from the Terminals avoiding the Air Trains.
-AirTrain to NYC Subway to Manhattan-EWR bus or train is another option and probably the most time consuming yet cheapest per person.