It may be possible to combine the trust funds into one account, but it may not be advisable. If the only beneficiaries of the trusts are the two customers themselves, then they would be considered to be joint owners of the trusts’ funds. See 12 CFR 330.10(f) (stating that if the only beneficiaries of a trust account are the account owners, it is treated as a joint account). If the customers own the funds in the original account in joint tenancy, they will both have rights of survivorship in the account. Joint Tenancy Act, 765 ILCS 1005/2. If the trust agreements provide for the same rights of survivorship, it may be possible to combine the funds into one account.
Even if that is the case, though, there may be more complications after both customers pass away. If both customers pass away, the trust agreements would govern the funds from each respective trust, and they may have different beneficiaries or tax treatment. Also, note that living or revocable trusts ordinarily can be amended at any time, by adding or removing beneficiaries or trust property, and the entire trust may be revoked at any time.