There's no reason to limit simulation to one level, nor to privilege "real" as any special thing. All reality is emergent from a set of (highly complex, or maybe not) rules. This is true of n=0 ("reality", or "the natural simulation"), as well as every n+1 (where a level N entity simulates something).

It's turtles all the way up.

Put another way, the simulation parent entities wonder if they're being simulated, so it's exactly proper for the simulation target entities to wonder, for exactly the same reasons. I suspect that in every universe, thinking processes that can consider simulation will consider that they might be simulated.

I don't know if they'll reach the conclusion that it doesn't matter - finding the boundaries of the simulation is exactly identical to finding the boundaries of a "natural" universe, and we're gonna try to do so.