I see what you mean... I had hard times when I switched from guitar to archlute (8 diatonic basses). I play an 8 strings classical since this summer and, of course, two extra strings seem much more easy than 8.
Anyway, there is only one way to solve the problem : pluck the extra bass strings ONLY with rest-strokes, no matter if you're ascending from basses to treeble (easy) or descending from treeble to basses (more tricky). This way, your muscular memory will gradually "learn" the different stretches and, in a matter of weeks of practice, you'll be able to pluck just the string you want. This exercice will prove to be efficient only if, while training your thumb with rest stroke, you rest i,m,a on the top three strings.
To my knowledge, this is the only way to practice : just as long as your thumb will be "lost in the air" with free-stroke, your right hand will never be able to learn the muscular memory of each stretch.
Hope this will help,