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I am using two gps modules with blunos (bluetooth arduinos) too get the distance between them and display it on a phone. This has worked great and on avg Im getting the expected 5m inaccuracy.

However I would like to get better accuracy so I was looking into GNSS with rtk. From my understanding RTK uses an unmoving base station with known coordinates and works out the error the GNSS of the base is getting. It then sends this error to my rover which corrects its own readings.

If this is correct (if not please explain how it does work) then do all GNSS in an area have the same error.

If they do, do I need the rtk with the GNSS or can I just use two and they will measure the distance between them accurately even if they don't measure their locations accurately

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Given two GNSS, there are errors that will correlate, and errors that won't. Because of the uncorrelated errors, you won't be able to get to 0 differential. Examples of correlated errors are things like ionospheric delay. Examples of uncorrelated errors are things like receiver noise. You'd also see significantly different results if one receiver is "seeing" different satellites (e.g. because of buildings obscuring reception).

In general, you want to avoid doing straight position correction - you need pseudorange (i.e. measurement to the satellite). For a pathological case, imagine you have unmodeled additional ionospheric delay for a satellite that is overhead the midpoint between two receivers. That will cause your reference receiver to be in error in one direction, and the other receiver to be in error in the other direction.

RTK is more complicated - the kinematics is more than just pseudorange compensation.

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