I thought it'd be much harder, but it's not. I find it hard to notice Francis' cues, but sometimes Elaine tells me when he needs to go. That makes it easier! :) Of course, finding the time to run him to the potty when he needs to go (even when he's willing to wait a minute or two), can be tricky. It was much easier to drop every thing when it was just one kid. Still seems like a good idea to spare a diaper or two when I can and keep his awareness up.
Also, I have a friend with a 5 month old who just started ECing. She also has a 3 year old boy who isn't potty trained. ECing the baby is going very well. Potty training the older one is a struggle and he's refusing to use the potty. She's wishing she'd started the older one on sooner so that he wouldn't be afraid of the big potty at this point. But maybe the little brother will be able to help the older one feel more comfortable. And it doesn't seem to cause any negative problems with the older one feeling behind or anything, which I'd sort of worry about. These kids seem pretty happy though!