That depends on whether they need service or repair work. For machines needing repair, it’s often not worth it, but why not?
Dual or double-cassette machines were generally the cheapest cassette decks of their time. Cassette deck maintenance can be some of the most labour-intensive work we do. How many cassette deck mechanisms are in a double cassette deck? That’s right, there are two, and they are not usually the nice ones to work on. See the problem?
This doubling of mechanisms often means doubling the already time-consuming workload, and that can mean more work than most of these decks are worth, to most people anyway. Ultimately, an owner will need to make that decision, but I generally avoid working on dual cassette machines for these reasons.
Viability vs Lovability
If both decks need standard cleaning and service work, then yes, it’s probably worth doing. If both decks need repair in the form of idlers, melted belt replacement, etc, this is generally something to avoid unless cost is not a concern at all. As mentioned, I avoid dual cassette decks for these reasons. The opposite is true of good single-deck machines of course and I still work on those and enjoy it!