The answer is that it depends on the type of Plan; who funds the benefits and who is responsible for making the claim determination. In most cases, the entity sued is the disability insurance company, since it is the party responsible for both making the claim determination and paying benefits. Although it may be designating as the Plan Administrator, suing the employer that sponsors the Plan but otherwise does not fund the benefits or makes determinations on claims would not be appropriate.
However, what if the employer self-funds the Disability Plan, but delegates the claim determination responsibilities to an outside third-party administrator? It may be appropriate in this instance to sue the employer, since they fund the benefits, and the third-party administrator responsible for making the claim determination. Certainly, the employer may attempt to argue they are not a proper party to the suit since they do not make the claim determination and it is delegated to a third-party administrator, and the third-party administrator could try to make a similar argument and say that they are not a proper party to the suit since they do not pay the benefits. If it is determined that one of the two defendants should be dropped from the lawsuit, it may be done at a later time.