You can not use the same pass for two people. Once a pass is put into the turnstiles, it can not be used again for 30 minutes, to prevent people buying a 6 day hopper for two people staying 3 days. You'll have to buy another pass. As to the birth certificate question. If you have a rather large 2 YO, it might be a good idea to carry a copy with you. The CM's at the gate are trained to look at children to see if any over the age of 3 are trying to get into the parks without passes. The CM's will ask children who in their opinion looks 3 their ages, and of course a 3 YO would answer they were 3, and you'd have to buy a pass.
Edited to add: I have had people question the age of my child. DD was 3 1/2 and I hadn't taken her pass out and we were immediately asked her age. Once I had gotten her pass out and showed the CM she had one, they stopped their questioning.