I just had twins this last year and my wife and I are going to be taking them on there first trip soon. I want to buy an extra seat in E+ so we have room to feed care for (and also to not crush someone else next to two possibly crying babies.. ) yadda yadda.. however I can not see how to do this?

I have had Premier or better ever since flying on UA and the idea of sitting in E- gives me the chills. How do I buy an extra seat next to us? Can I get our two seats and then call up UA and pay the extra for E+ for the third seat? It sounds complicated, and slightly scary for IRROPS.

Anyone had experience with this kind of thing?