This sounds so familiar, I really sympathise.
dd was born at just turned 34 weeks - she was a good weight and pretty healthy but it took until 37.5 weeks for her to breastfeed well enough to be allowed home. (And then another few weeks at home before feeds were no longer a struggle). Hopefully your girls will be quicker than mine, but it does seem common for it to take until 36+ weeks. At the time the 24 days in hospital felt like forever and I was desperate to get DD home ASAP, but with hindsight it would have been better for me if I had accepted she was likely to be there a while and there wasnt much I could do to speed her up. Hang in there, they will get it in their own time. Do try to take care of yourself as much as possible in the meantime as you will need the energy once they get home.
Re trying at every feed - I would say yes if you want to, but don't try for too long if it seems unlikely to work that time, as youll get Frustrated and theyll get tired. You may find they are better at some times of day (dd was better in the evenings), if so, maybe focus on those times and let them be tube fed at other times. Or you could try them on the breast at at alternate feeds (especially as you have twins). From my (limited) experience, I don't think trying more often necessarily means they get the hang of it quicker - it's down to when their instincts kick in and muscles develop enough, more than practice.
Re milk production - yes do hire a hospital pump if you can, they are so much more effective. I would say pump every three hours in the day and once in the middle of the night (some people say every three hours at night too but I think it's more important you get some sleep!). If you can pump next to their crib in the hospital, or look at photos of them while pumping, that is supposed to help. Eating lots helps too I think, if you are able to find the time.
Best of luck, hope you can bring them home soon.