My first, we don't actually know if she was back to back, but I felt every contraction in my back, and CTG monitoring kept being lost despite me not moving. Either she turned in labour or it's just the way I feel contractions, but as no-one mentioned her being sunny side up when she came out, I assume she was in correct orientation.
My second however.... He was definitely back to back. He'd been breech less than a week before, so to ensure he was still cephalic, I had a scan when induction was started. A few days later, when there was space on delivery suite, I then had a bedside scan prior to the ARM. Then another when the call to have a C-section was made. When he was extracted not only was he back to back, but he'd tipped his head back as well. The consultant said he was stuck and would have been undeliverable vaginally.
Similarly to my first labour, I felt every contraction in my back, and felt the need to increase pain relief at 5cm again. I was far more mobile and tolerated labour better with my second. The key difference between the two was the length of labour to get to those 5cm. With my first it was about 6 hours in (from 1cm). With my second it was after 10 hours (from 2cm). I chose opioids with my first as I wasn't keen on having an epidural. I went straight for an epidural for my second. It had taken so long with no end in sight. I was very demoralised (it was supposed to be "quicker and easier" after all), tired, in pain, and couldn't take it any more. After pain relief I progressed to 10cm with my first after 5 more hours. With my second I remained stalled at 5cm after 3 more hours. Baby was starting to show signs of distress. As I was pain free, I would have been happy to continue labouring. But in my heart I knew it was over and opted for the C-section. I was still 5cm an hour and a half after that when they checked shortly before starting the C-section.
I don't regret trying though. I've heard that most babies back to back at the start of labour (~30% are) will turn during it (less than 10% are still back to back at the point of delivery). I was just unfortunate. If I were to ever have a third, and knew baby was back to back, I would still attempt a vaginal labour. Though I might have an epidural a bit earlier.
While I know back to back is viewed as a variant of normal, and it's just the head down part being important, the cynical part of me thinks that the reason position isn't checked upon presentation to the hospital is because a lot of women will opt for a C-section rather than experience a back to back labour. And hospitals really don't want more C-sections than they already have.