Not sure what p1 and p2 is but I've taught two different mixed year 1/2 classes in different schools and found it great fun. Just think about making sure you've got a two year planning cycle and differentiate a lot!!! You'll probably find the spread of ability will be a lot less than you think it will be. Good luck.
At my DSs' school both P1 and P2 are taught the same subjects together (on a two-year cycle) but are given different written work and homework according to their ability. At this age I don't think it's so much of an issue as there is such a big spread of abilities by age. My DS2 is the oldest in the year and two of the other children are 51 weeks younger and the difference in maturity and ability were huge in P1.