You are doing an amazing job Fringe, you should be so proud of yourself.
I never got the hang of tandem feeding, even with a full day session at the hospital, one-on-one with a lactation consultant (and a few midwives coming in to help constantly as well!) In the end even the LC said, "At this stage, don't bother trying."
Both had problems latching on, combined with DD suffering from bad reflux making the whole thing really hard. Latching on never improved for either and DD reflux didn't fully clear up until about 8 months. She'd take ages to feed, then scream and arch and then throw it all back up.

It wasn't so bad on the bottle as we could sit her up more to feed rather than lying her more flat for the breastfeeding.
I persereved for 3 months, determined to make it work. I'd breastfeed one while someone else bottlefed the other EBM (and some formula) (DH had the first 2 months off with me, MIL and my mum helped the next month) and then I'd express in between
every feed... over a litre a day in the end!
It was exhausting and by three months I knew I'd done everything I could and had given my babies the best start in life I could. I was miserable, sick of having people over all the time (personal space had evaporated!!) and depending on their help.
It was the best decision for me at the time and I don't regret it. You need to decide what is going to be best for you and your family. Don't beat yourself up or think you haven't done a fantastic job because you have. Breastfeeding one/two/neither, whatever - just do what is best for your family.
I know of one twin mum who breastfed one baby til 12 months while the other was FF from very early on - that's what worked for her family and of course there is no difference between the twins. Neither complained!
Good luck with everything.
ETA: It really irks me when people give advise of their own agenda. Fringe asked about breastfeeding
twins specifically, not one baby. There is a huge difference, and it doesn't help giving the standard old "breast is best" advise cos the question isn't a regular breast feeding question.
Don't get me wrong, I completely agree with the 'breast is best' general rule, but I have been through the long, hard and at times extremely difficult process of trying to breastfeed twins so I know that it's a lot more complex than that.