Yep, and even here it's not an "independent" duopoly (basically walmart and target in the same city) but switching from one to another means buying a new device, sometimes for $1k+, and your whole ecosystem.... if we're talking about developers here, also learning a new programming language and doing all the work again.
Yeah, these are really independent markets, in the same way that a store can be a monopoly even if it is only a monopoly "West of the Mississippi" with another not-actually-competing store in the East (which might or might not be a monopoly there; in the case of mobile app markets it is, and the result is really two monopolies, not one duopoly).
