Can a baby guinea pig get in the same cage as the one you had for 2 years?

Yea

Improved:

This is a possibility, but is not always the case. Many guinea pigs get along just fine; no matter what litter they are from. On the other hand, two guinea pigs may fight whether they are from different litters or not. You should always be careful and monitor interactions between two guinea pigs who are just meeting. Once you are sure they will not fight, you can let them be together in a large enough cage without constant supervision.