TT, I always find I get way more letdowns tandem feeding than individually. Something I noticed when we tandemed all the time was once one girl was latched and sucking, I'd start hosing on the opposite boob which got other baby sucking like crazy. Pretty much because of the letdowns, as soon as one baby would start sucking, it would get the other moving too which is awesome for boosting supply as it means both boobs are getting constantly stimulated.

Don't know if the effect is any different tandem feeding two bubbas of different ages but that is my twin experience