Most data vendors grew up in one market. The equity houses added crypto later, the crypto houses may never add equities, and in both cases the seams show in how it is sold: a separate product, a separate key, a separate integration.
For the buyer that split is pure overhead. The questions do not respect it.
The questions that cross the line
Is the index tape leading crypto today or following it. Is realised volatility elevated in both or just one. When risk came off, which book moved first. Those are ordinary questions and they are unanswerable with two disconnected feeds that use different symbol conventions and different freshness semantics.
Even the mundane version matters. If the equity map reports its age in a field and the crypto map reports it in a header, your code has two paths where it should have one.
What a single key buys you
One authentication path. One error vocabulary. One freshness contract. One place to look when something is wrong. The integration cost of a data feed is mostly not the happy path, and every one of those duplicated pieces doubles the unhappy one.
It also makes cross-market tooling trivial rather than a project. A screen that ranks both books by the same stretch measure is a few lines if the two share a shape and a rewrite if they do not.
What to ask before buying
Does one credential reach both markets. Do the two use the same field names for the same concepts. Do they report freshness identically. And what happens on a request for a market your plan does not include — a clean, named refusal, or something ambiguous.
The last one tells you whether the two products are genuinely one system or two systems behind one invoice.