If E1 and E2 have the same value, E, then the current in R will be E/R.
IF E1 and E2 don't have the same value, then the effective voltage applied to R will depend on the resistance of the wires connecting E1 and E2 together.
In the real world, the wires connecting E1 and E2 would probably be copper wires of fairly low resistance, so that if E1 and E2 were of differing voltages, a large current could flow in those wires. Depending on just how much current E1 and E2 could supply, the connecting wires could suffer effects ranging from just getting a little warm, to melting the wires. Damage to the voltage sources might occur.
Not a good thing.