The recommended 2 to 3 week wait is for quarantine purposes- to avoid a new arrival with an illness passing that along to the guinea pig you already have. As they've already met and have contact with one another, there's not much point in doing further quarantine. Based on that, if you want to try housing them together, go for it. There are some stickies that talk about how to introduce new guinea pigs that should be helpful. It's good to meet on neutral ground (the last time I did intros, I actually laid old towels down in my bathtub- they certainly hadn't been there before! It is also a good idea to scrub down the cage they will be living in completely and move things around. A lot of guinea pigs will be happy to meet a friend on neutral ground, but less happy to have a friend move into their house. Removing all the scents tricks the resident pig into believing this is a new space that he/she is less likely to be territorial about. Hope this helps!