If you are going with two different cabs, I would send the cleans to the biggest one and overdrive/distorted to smallest one. Big waves (lower frequencies) need bigger cabs than smaller waves (higher frequencies)... and higher frequencies can get away with smaller cabs. Distortion - higher frequencies. Smaller cab on top.
YMMV, try it either way and see what fits the tone in your head.
I get your point, and I agree that is the conventional way. I think most players look for a full bodied clean tone with added spice, and that makes sense. I'm not that sensible.
I already have the tone I'm looking for, more or less. It's a fat, mid heavy, overdriven tone with a solid low end (it goes through two tube overdrives). I just want to add a pinch of clean spice to it. The Ashdown 410 does that, but not very well. It's a dull, lifeless, boomy cab with a hollow sound to it in this setup. Might sound great to someone else in a different room with different gear!
My theory is that a good 210 will offer a tighter experience for what I'm after - a (relatively) subtle clean boost in lows/low mids. Maybe cut most of the highs for emphasis? Could have been a 112 or a 212, but I do like 10s. Especially Barefaced 10s. Also, it will never come close to the volume coming out of the 610.
Anyway, I appreciate the input, just felt the need to clarify. Progress is never a result of doing stuff the conventional way. (Then again, it's by no means a given that this will equate to progress.)