Most vets spay buns around 5 months of age. If it were me, I would probably spay them both at the same time just to only have to deal with it once, but that decision is yours. They may prefer the company of each other, but they will be sore and groggy for a few days afterwards, anyhow, and will spend a lot of time resting.
I’ve never had females pull their stitches out. I don’t think that’s too common. Just keep an eye that they are not over grooming or picking at the stitches. And make sure to check their incisions everyday for redness, swelling or bleeding. Most buns heal up just fine, however. If the worse did happen and they pick at their stitches, your vet can fit them with a cone.
Be prepared for your girls to be sleepy and unwilling to eat after surgery. You will want to make sure your vet sends pain meds with you as they will probably need some for a few days. A good idea to have Critical Care on hand in case they are too reluctant to eat. Keep their area very clean after surgery. It’s a good idea to give everything a good scrub while they are at the vet so it’s nice and clean when they get home.