I used an avent hand pump for occasional expressing with my twins (just for some EBM for the freezer). When I needed actual large volumes of milk (ie when one of my twins was in NICU) I borrowed the single electric pump from the hospital and made sure I expressed for the same time on both sides. A double one would have been helpful in this situation, but I would hire rather than buy one in that instance.

TBH though I only needed to express when my son was apart from me, and the rest of the time it was just like with a single baby in that I expressed a little bit regularly to build up my frozen supply.

WRT tandem feeding, it is really hard. Some people get it straight away, but if you find it is not working then don't be afraid to feed them individually. With the time it takes to attach 2 babies for a tandem feed and keep them in place, it is honestly just as fast to feed them one at a time. I tried tandem feeding for over a week but my babies were not putting on weight (and one was losing weight). Switched to single feeding and everything improved, I was able to feed them for almost 2 years.