Well, I have used 2 targets, to avoid the time spent in looking thru the spotting scope to locate the first of the two. I really think this may be unnecessary, though, as that time taken is minimal,but I do try to get them both off as soon as I can acquire a new sight picture. As far as the time between first 2 and second, etc, I have tried a couple of ways. First, let 'em cool completely. Second, go ahead and let the next shots off, just giving enough time for the barrels to reach the same temp, checking to verify they aren't hotter than a warm day hunting. I can't tell any difference in accuracy! Course, maybe that's because I can't hit anything anyway! Now, keep in mind I'm not shooting the really big nitros- 9.3 and 577-500#2 are the biggest, and that surely can make a difference.
My 2 cents, Doug