If they are both in your water now and wet, they can't be reused so keeping both for these four months won't do any harm. If they haven't been used then save one for next four months. Right now I am using a regular N2 cartridge. I have been using the Sun Purity and have used the Spa Frog, anther manufacturers version. The spa stores in my area tend to sell all three for the same price, so I take my pick. I may try ordering in quantity online next.