[utoku]

A bit of clarification, I think the orbiters are in sight of Earth for most of the day.

It is Curiosity that is not in sight of the Orbiters, again because of their low orbit.

[danielweber]

I'm not sure I understand. Gale Crater is pretty near the equator, so any equatorial satellite would pass quickly over it, but many many times per day.

[uvdiv]

You answered your own question: both satellites are in polar orbits. They pass the equator many times a day, but at a different longitude each time.