I thought I would need one by now, but I have learned a lot about the body changes in a twin pregnancy. Things happen so fast (growth), that there are times early on that you think "how will I make it if it's already like this???" One huge thing that is happening is that your body is releasing the hormone relaxin in high quantities in order to help your body with it's rapid changes. Great stuff, but it makes for bony misalignment. Have you ever been to a chiropractor? It can be intimidating at first, but for myself it is essential. I was going once a month (yoga in between to keep things where they needed to be), and have decided now at 30 weeks to start going every 2 weeks. It is well worth the money to get the education about your body and the relief. There are some "added benefits" you can get from chiros as well. My chiro does craniosacral work and it's so gentle and effective, I bring my 3 year old for it at least once a year. Also, there is the "Webster technique" that is known for its' effectiveness in getting the babes to go head down, as the pelvis is more open for them to do that.
The longer you can wait to get the sling (or use it), the better. You want to keep muscle tone, or that can be challenging post partum. However, if you have to be very physical for a job, that might be a good choice. I'd try to find a chiro and talk with them about it. If you have a MW, they probably have good ideas. Even if you don't, I'm sure a MW would be open to a referral call about one.
I am happy to answer any other questions about this (as far as my scope of knowledge goes), as I am a massage therapist and have a lot of good to say about massage, chiropractic and acupuncture during pregnancy.