well, the only reason for developing different films in the same tank is when i know that the same amount of time/temp/agitation works fine for both films. you might consider pulling the delta so 8.5 minutes would be an applicable dev time.
you might have to run some tests in order to find out which e.i. is appropriate for developing delta for only 8.5 minutes. or run some tests in order to find out how you should expose, under which kind of lighting, pan f for developing it for 12 minutes. or you might look for two different films which require the same dev time (e.g., according to the mdc, hp5+ @400 and ilford pan 100 @400 iso both require 12 min in 20°C xtol 1+1).
or you could simply develop films separately, respectively 2 of the same kind at a time.