Had to RTFM to even guess, and the very "Marshall" way of explaining things still left me confused.
BUT, as anything Marshall IMO is done for cavemen, by cavemen, I think I found the answer on page 5 of http://www.drtube.com/schematics/marshall/ibs.pdf
If I understood that correctly, You patch the hi-/lo-outs to the power amp ins.
That should defeat any internal routing.
Newer amps AFAIK
use internal routing and switches for the task, but it seems that these Marshalls don't.
Even though You're going to bi-amp the 115+410 combo, prepare to recieve tons of criticism on that particular combination
But then again, I rock 2 EV TL505's, so who am I to criticize anyone