It would make no sense for you to take something that makes you pass waste more often (alli) and then take something on top of that, so that you WON'T go. (Immodium)
It would be like wearing SPF50 Sunblock in a tanning bed.
If you were going to take Immodium to try and stop the effects of the alli, then the best way (and easiest on your body) would be to not take the alli at all.