With the life estate you have three values that can be determined:
1. The value of the Life Estate
2. The value of the Remainder interest
3. The value of both of these held jointly, which is the Fee Simple interest.
The fee simple interest does apply. The two can get together and sell the house in fee simple title if they both agree to sell their interest together. When doing an appraisal for such a sale, the fee simple interests are not hypothetical. The two can also come together and take out a mortgage jointly on the fee simple interest of the property. The fee simple interest, when they do come together, applies. In such a case, if the house is foreclosed on, both parties are responsible for the mortgage and both interests are foreclosed on with the lien holder taking fee simple title.
If your assignment is to appraise the fee simple interest in a house that is held in a life estate and remainder interests, as long as you disclose it in the report, I see no issues as both (or all) parties, not individually but when they come together, do have the right to sell and finance the fee simple interest in the house and the fee simple interest, when they do come together for such a purpose, is applicable "as is".