As with all legal questions, it depends…
First, do they both have the same level of disability? Will they both qualify as disabled under the Social Security Definition? If so, then you can setup one third-party Supplemental Needs Trust (Special Needs Trust) to serve both of them. If one of the twins should pass before the other, the money will stay in the pool to serve the needs of the other twin.
If they have differing levels of disability, then you would be better served by setting up two independent Supplemental Needs Trusts. You set both trusts up to pour the money into the other trust if something should happen to one of the twins. That way, the money keeps on helping the twin that needs it longer.
By using a third-party trust, when both twins have passed, you decide where the remaining money and assets in the trust go.
If grandparents have purchased life insurance and made the twins the beneficiaries, you want to make sure that it pays out to the third-party special needs trust, not to them directly. By paying to them directly, it might disqualify them from means tested benefits. The same if you have life insurance you are using to fund their care. Pay your life insurance to the third-party supplemental needs trust, not to the twins directly.
It might also be a good time to think about setting up a first-party (d(4)(A)) trust for both twins. One requirement for this type of trust is that they be under 65 years old. By setting up this type of special needs trust, you make sure that if they do come into money of their own, there is a place for it.
Another option is to open an ABLE account for both twins. Even though it has a Medicaid payback provision, the account is there for small amounts of money.