It is not a legitimate conclusion that they MUST BE 2 separate groups.
A literal rendering would be: "And I saw thrones, and they sat on them, and judgment was given to them, and the souls of "the having been beheaded ones" because of the witness of Jesus and because of the word of God, and who not worshipped the beast nor the image of it and not received the mark on the forehead and on the hand of them; and they lived and reigned with the Christ a thousand years."
The word "hoitines" appears only once and that in conjunction with the clause about the mark and image. The first "who" is understood and is literally, "the having been beheaded ones." It strengthens the rendering of the versions that identify the two groups.
Had they wanted to clearly identify two groups they would have said "and ALSO the ones who."
As it is, the king james actually preserves the fact that there is no "who" in the first clause but there is one (which) in the 2nd clause.
To conclude two groups is to make an assumption.
Revelation 20 4 And I saw thrones, and they sat upon them, and judgment was given to them,and the souls of those who have been beheaded because of the testimony of Jesus, and because of the word of God, and who did not bow before the beast, nor his image, and did not receive the mark upon their forehead and upon their hand, and they did live and reign with Christ the thousand years; 5 and the rest of the dead did not live again till the thousand years may be finished; this [is] the first rising again.
The wording is awkeward xzings..but it still can be two groups..Those that receive the judgement the the and