It is a really tricky situation, but it semes like Markström is in the beginning of his prime, and I think he could keep playing at this level for another four years at least. So I think we should try to sign him to a 4-6 year deal @ circa 5,5-6 which I think is reasonable. Then Demko will be able to develop this year and next year as well and we could have a situation similiar to the Luongo and Schneider situiation, where we have two number one goalies.Then one of them can probably be dealt at the draft in 2021. At this time, both will probably have a high value, and they could be dealt to whoever needs a goalie at the time for potentially a 1st rounder or more depending on how they have performed. If Markström is able to maintain this level of play it would not scare away anyone from taking that contract for three years, and if Demko can get ca 80 games until then with good numbers there will be an interest for him as well.
During this time we will see how DiPietro progress as well, and his numbers so far looks very promising and he could then step in as back-up to whoever is decided to be our number one.
Even if it is a tricky situation, it really is a luxury few teams have.
Should add that I haven't really considered the expansion draft and protection rules in this analysis