you just do it, dont try and hide the fact that your feeding, the 2 yr old will want to check it out just say that you giving bub their food etc, they may even want to have a try etc, the worst thing you can do is make a big deal out of it, some of mine have come up for cuddles at the same time etc, I just do what they feel they need so they dont feel left out etc :)
I agree with stacey. I didnt make a big deal out of it, when my little one would get curious i just said mommas feeding baby and he was fine with that and most times would sit with me while i fed baby.
My kids have 'fed' their stuffed animals when I'm nursing the baby. It's sweet. They want to be right in there... I ask them to back up and they usually do. I work and they're able to help give bottles which helps.