IANAL, but I would think that the arrangement could also potentially hurt the electors as well, as it ties their fate to that of the GOP chair and takes any plea deals or hopes of lesser charges off the table. It would appear to be to their advantage to have different lawyers.
tl;dr The judge views the Georgia GOP chair as being in serious legal jeopardy, but the others not so much. By having the same lawyers, they can coordinate their defense, benefiting the GOP chair, but harming the prosecution because the electors can't be turned.