I'm tandem nursing my 7 week old and my 18 month old and it's going well overall. Last night, DS (18 mo) was trying to nurse but was getting very frustrated because he wouldnt get much milk, there was no let down. It had happened several times before with him, but never with DD (7 weeks old). While nursing DS I started thinking about how I always get a let down with DD and for the first time in a while, I had a let down for DS!
At the next nursing session for DS, I thought about DD's face and smile, her smell and all that... And again I got a let down. DS was super happy. But it left me with a really weird feeling of "cheating" on my toddler. Kind of like if i was having sex with DH, but having to think about another guy to be able to really get in the mood. I thought that was both funny and weird ;)