Catman, I hadn't heard of the two hour time period, but it makes sense. Because the antibiotic wipes out all types of bacteria, both good and bad, so if taken too close to the time of the probiotic, might work against the probiotic.
The probiotic doesn't work against the antibiotic, because it replenishes only good bacteria, not bad bacteria.
After an antibiotic is administered and wipes out a good deal of bacteria, it is the bad bacteria that is most likely to grow back, first. So taking a probiotic helps to prevent this by filling the gut with good bacteria before the bad bacteria can take hold and cause things like D.