Actually, I holding one in this precise moment (inserted about 3 or 4 minutes ago). Never tried more than one at the same time, and one for me it's like setting a alarm clock - I hold it for about half a hour, then it's a big urge and expelling for another half a hour.
I've just tried now more than 1 Dulcolax. 45 minutes ago, I've inserted 2 suppositories up my lovely tight hole and started the stopwatch :D
At the minute 28, I came to the toilet (yes, I've a laptop) so I'm expelling right now while writing this :D
Conclusions: took the same time to produce the effect as if I used only 1, and the urge to go it's also the same... maybe even less than some other times with only 1... About the time they took to produce the effect, I think it's normal to be the same - 1, 2, 3 or 10 suppositories will take the same time to melt. But for the effect, I was expecting it to be stronger...