The only way I know to disable multi-tasking in AMOS Pro is to use Forbid from Exec library(?), and re-enable multi-tasking by using Permit (also from Exec library).
They have improved the speed of some of my AMOS stuff in the past, but there's a chance that AMOS may crash.
AMOS (and AMOS Pro) generate the copper in realtime, which is used for thing like the Rainbow commands, setting up screens, sprites etc.
This all makes it very slow and as AMOS hits the hardware directly (from what I remember) it is not OS friendly, therefore not very multi-tasking off friendly either.
Hopefully bruceuncle is reading this and will confirm what I said.... and correct the bits I may have got wrong