No experience of back to back but I've no doubt it's possible if you have the right mindset for it. I've found hypnobirthing techniques really helpful.
Ds1 was unidentified breech, stopped progressing at 7cm after 24hrs labour and ended up as emcs. No pain relief other than the epidural just before the emcs.
Ds2 home water birth, 3 hours start to finish and wonderful. No pain relief.
DS3 planned home birth, wasn't descending, had to transfer in (an hour from where we live!), Then 2 more hours pushing hard and shoulder dystocia. Severe haemorrhage 5 hours later with internal and external massage required to contract my uterus. No pain relief.
DS3 is the only birth where I felt like it hurt. Crowning Ds2 didn't bother me; I didn't even notice it with DS3 as the sensations that were most uncomfortable were higher up.
I think there are 2 main aspects of medical pain relief in labour which are often overlooked
1- they interfere with your bodies production of endorphins (your own pain relief) and so reduce your "natural" ability to cope. Endorphins are also inhibited by stress and fear so feeling calm and safe means being more comfortable.
2- opiate based pain relief (in between gas & air and epidural) can have negative side effects for your baby. Both opiate pain relief and epidural CE with an increased likelihood of other interventions (forceps, cs etc).
Also there's nothing stopping you planning not to have pain relief and then changing your mind.
With all of my labours I explicitly stated that I did not want to be offered pain relief- that I would ask for it if I wanted it. I'm really glad I did with ds3 as if someone had offered me an injection to make it stop I would have said yes but that really wasn't what I wanted and I managed just fine without.
If you want to try without you should do it, you might be surprised at what you can do.