Does it work to do shorter nursing sessions? This worked well for us during the pregnancy when it was so uncomfortable- I'd mentally sing the ABC song, maybe even a couple of times through, then we'd be all done. I'd also try to entice her to do something else by suggesting that we'd have some time to do some other favorite activity (read a book, play with her dinosars for a few minutes, etc) if we only nursed for a short time. Sometimes she wouldn't go for it, but often she would- and she was a pretty dedicated nurser as well. It was a lot more tolerable for us both than me just saying no (which I also did at times too). I got the suggestion from my LLL group, so it's something that worked for some of them, too.
If nothing else, I know it's helping me to know there's someone else out there that doesn't love tandem nursing- and can totally understand it causing more antagonistic feelings. I actually find myself thinking "you're big, why are you needing so much?!" Fortunately, I know she needs so much because she's still a child & I haven't actually said this, but it does often center around these requests to nurse more often.