One of the advantages to a m/p is both sides can wear the same gear provided the priest has high enough level to wear the exact same items as the mage. This is another incentive to keep your priest as close to the mage as you can in level, over the other option of going all out on the mage and leaving the priest far behind. This way not only is your mage going to benefit better from the priest secondary skill levels, but there would be no need to swap much if any armor or weapon when switching your class. Makes it much more efficient for making/buying your gear for this setup too, which is my favorite reason for going with a m/p. R/s is another setup that can work like this very well. If you plan to be a healer while on the p/m setup, you will need another set of gear stated for that though, but for me, just having the one set of gear has worked out great in keeping up my priest side with the mage level.