12 volt batteries

No real problem as long as both batteries are indeed still functioning fine. Ideally, mixing batteries of different age or brands will not give you the best results over the long haul, but for short term expectations they will suffice. Biggest problem is they will charge and discharge at different rates with the weakest battery of the pair causing the stronger battery to languish in overall performance. If one battery is particularly weaker than the other, using Blue Sea or Perko marine battery selector switch could be incorporated to keep them isolated until they are needed. Otherwise, if both are healthy and strong keeping them paired up would take advantage of Peukert's Law.