I'm not sure exactly what you're doing in the split transaction, but I add up items for each category all the time. I enter the total amount of the split in the main line, tab down to enter the first category, and in the amount field I hit, say $5.00 and then the + sign until I've entered all items in that category on the receipt. I do the same for each split.
Does that answer the question?
"It’s still all about the method. Fancy Cloud Sync algorithm aside...the software is there to help you become more aware (Rule One), anticipatory (Rule Two), flexible (Rule Three), and secure."--Jesse's blog, A Method to Your Madness