hey, sorry but it’s not possible to do in a single roll expression right now, using the L roll condition won’t work because each roll condition applies to one dice pool only. You’ll have to roll two times and then disregard whichever is lowest yourself.
I do have plans for improvements to the roll expression system that would allow what you’re trying but they will require some pretty substantial changes to how it works, so it probably won’t be part of the next update, sorry!