Developers had been writing multi-platform code for 10 years already. When you have API calls isolated from the rest of the code it takes a couple man/months tops to target another API.

The hard part of the previous generations multi-platform had been in getting to the same level of performance. E.g. 360 had a pretty weak CPU and a superior GPU compared to PS3 so the distribution of things done on CPU and GPU was completely different between these two targets.