I just finished a simultaneous countdown in both US and UK. There's no reason not to schedule both, particularly if you're running mailings at the same time. The dates don't have to be identical but likely you want them to be close. The rules limiting the timing are the same and there is only one KDP Select enrollment period. Some minor details of the Amazon site vary between UK and US -- it's harder to find the countdown deals list in the UK and you can't (easily) see the pricing info when you access the UK site from the US. On the other hand, it's easier to get on a bestseller list in the UK. In the US, I had a spike of sales at the beginning, but in the UK, it was at the end after a handful of sales put the book onto a category list.