It may not be illegal to have twins *at home*, but it may be illegal (as in beyond scope of practice) for licensed midwives to attend the birth in some states. (If it is beyond scope of practice, then if anything goes wrong, the midwife could be prosecuted.) Or it may be outside their malpractice insurance coverage. Either way, it is a huge risk for the attendant.
For example, my midwives in Ohio told me they could not legally be my primary attendant if I was outside normal gestation (before 37 weeks or after 42) or if the pregnancy was not singleton, among other things. I never checked to figure out if this was in their licensing laws or a malpractice insurance issue. It didn't matter because it would have had the same effect.
But like I said, I'm pretty sure neither of those things is an issue in Tennessee.