I have done this with two 2/3A NiMh 6 volt packs. I needed capacity and there were space limitations. I started with two new identical packs and with both packs at the exact same state of charge/voltage. I used a heavy duty Y harness and placed the connectors to each pack so that I could monitor individual voltages from time to time. Combined capacity discharge test were ran about 4 times and the results were very good. I did this for several years in that plane and never had a problem the packs are now in two different planes. There may be problems that I didn't foresee or experience.