
Wouldn't it depend on where you were up to with progesterone testing (just one test or multiple tests to identify the time of actual ovulation), whether you were travelling a long way, and how stressed the bitch is?
Last year when I drove my bitch to Poland, which turned out to be a bit short of 3000 miles, she had bloods done on the Friday, indicating we needed to be there for Monday, the vet reckoned Tuesday-Thursday as the window for mating (Tuesday and Thursday if doing 2, Wednesday if doing 1); got there Monday night and just going off my own stud dog's behaviour with her before we'd set off had her mated Monday night (day 8), then did a second progesterone test on the Tuesday night which indicated ovulation (day 9), mated again and the third and last time on the Thursday morning just before setting off home (day 11). No doubt we didn't need 3 matings, but I'd asked for 2 and the stud dog owner offered a third just in case :-)
We had a bitch over from the continent for my dog a couple of months earlier who was allegedly ready, but the stress of travel really set her back, and she was eventually mated on days 19 and 21 (her "normal" cycle would have been days 14 and 16). So in answer to your question I would always prefer 2 matings, because there seem to be so many variables involved for both stud dog and bitch.