Connecting different batteries in parallel - how to calculate the voltage?
First of all, these batteries are not ideal!

Convert both voltage sources (with thier resistor) to the current current source equivalent.
The left one will be a 1A-source with a 1 Ohm resistor in parallel, the right one will be a 1A-soure with a 2 Ohm resistor in parallel.

Then you can add both current values to 2A. With the 0.66 Ohm resistor you calculated, the voltage will be 2A * 0.66 Ohm = 1,33 V.

Sounds pretty much like homework...