2 neutral wires to one connector
Do the backstabs accept 14 AWG only?  Are the bathroom receptacles on the circuit?  You cannot use the lamp, bathroom receptacles must be 20A and 14 AWG wire is not legal on 20A circuits.  If it has screws, use them - backstabs are bad news anyway.  

Ok, so get a 6 inch length of correct size Romex.  Strip it down for the hot, neutral and ground wire.  Use those to connect to the single terminal on the lamp.   This seems like it makes things worse because now you have 3 black wires and 3 white wires to connect instead of only 2.   Get wire nuts or other type of splice connector which accommodates 3 wires.