Funnily enough I had this exact scenario in one of my last sim checks. We had just departed and GEN 1 overheated and had to be disconnected. After the ECAM etc I got the FO to start the APU. Funnily enough the APU failed to start twice. At that point I elected to return to departure airfield as we were only 20 minutes out and still had 2 hours plus to destination. Checkie was not impressed, he could not understand why I went back considering I still had the RAT and the EMER GEN that could run off that as a back up to the one remaining GEN. I simply stated that after one GEN failing followed by the APU failing I felt the safest option was to let an engineer look at the aircraft, I also stated that I didn't consider the EMER GEN as 'another' GEN as it is there purely to get you out of the '!!!!!', not to allow you to continue normal ops. At the end of the day, this is a bit of a 'gut' call, I would still land at the nearest suitable and get this sorted out, the checkie would still carry on to destination happy to rely on the RAT.