According to the current understanding of cosmology and the theory of general relativity, galaxies can indeed have an inertial frame of reference in common even if they are moving away from each other at speeds greater than the speed of light as indicated by their redshift (z > 1.4). This phenomenon is known as the "metric expansion of space."
In general relativity, the expansion of the universe is described by the concept of spacetime itself stretching or expanding rather than objects moving through space. The metric expansion of space is a feature of the universe on large scales, where the distances between galaxies are increasing over time.
The expansion of space is not constrained by the usual limitations of special relativity, which sets the speed of light as an upper limit for objects moving through space. The speed of light limitation applies to objects within spacetime, but the metric expansion itself is an expansion of spacetime, and it can exceed the speed of light.
As a result, two galaxies can be moving away from each other at speeds greater than the speed of light, as indicated by their redshift, without violating the principles of special relativity. However, it is important to note that this apparent recession speed due to the expansion of space is not a velocity in the traditional sense of objects moving through space but rather a measure of the expansion of the fabric of spacetime itself.
So, even though two galaxies may have a relative velocity greater than the speed of light as inferred from their redshift, they can still share a common inertial frame of reference in the expanding spacetime of the universe.