If two reference frames are not moving with respect to each other, do we take into account their distance apart to determine simultaneity of events?

The lighting strikes are simultaneous in your scenario. The observer may not observe the flash hitting their eyeballs/detectors at the same time, but when they work out conceptually when the lightning flashes had to have occurred in order to cause the observed results, they come out to the same result: the lighting strikes occurred at the same moment in time.

In fact, to be extremely formal, these observers are actually in the same frame, but they are measuring with different coordinate systems. The vectors describing space, time, and events are the same vectors for both observers. They may assign them different numeric values. One may assign a vector for the tree's position as <0, 10, 0> and the other might assign it as <5, 23, -9>, but the vectors describing the physics of the problem are the same.

Were one of the observers to be in relative motion to the other, some of the vectors would actually be different, not just measured differently. This is where we could arive at the result that the two lightnning strikes were simultanious in one frame, and not simultanious in another.

This, by the way, is a tremendously frustrating subject, even when we're not accounting for relativity. Very few people properly handle the difference between frames/vectors and coordinate-systems/coordinate-vectors. More than one prototype vehicle has been flung unceremoniously to the ground by mistakes like these, So don't be too bothered if it takes a while to get it.