I dont think you will be able to keep tolerance down if you are getting high every time. In order for the incomplete cross tolerance to have any effect you probably have to have a month at least off the specific substance.
I have had the idea for a taper though where you aren't getting high you could possibly swap back and forth every month or so and be okay at a lower than 'equivalent' dose because of the cross tolerances, when I switched from methadone to morphine I only needed about 75% of what they considered equivalent so perhaps I could have kept switching back and forth lowering doses every time who knows