Do not put anymore fish or creatures (like crabs/snails/shrimp) in that tank, they will just contribute to the waste load. I think the average lifespan of a guppy is 2 years, but I am not positive. If there is no way you can find another tank for them (try to look for a friend who has a larger tank), then you just have to wait until they both pass on. If one dies, do not put a replacement guppy in.