We don't know the construction of the cable, so it's not possible to be prescriptive. I presume the tonearm end is the usual DIN derivative?
In essence, if you have a pair of coaxial cables and a separate earth, the two signal pins for a channel of the cartridge find their way to the centre and screen of a run of coax, then the outer of the coax finds its way to 0v inside the preamp, which provides screening. The earth wire is then completely separate.
The XLR terminated cable probably has two runs of screened twisted pair. So now the cartridge signals run down the inner pair into the hot/cold of the XLR and the screen connects to the other XLR pin, the preamp connects this to 0v or earth according to taste. The arm (possible TT too) earth is separate.
To reterminate the XLR cable with phonos you can either run the screen and one of the inners (you want the one attached to the blue or green cartridge pin to preserve absolute polarity) to the phono shell and the other inner to phono centre, or you can separate the screen and join it with the other channel and the arm earth wire attached to the preamp earth terminal. Or perhaps it should remain unconnected.
I would go with the three earth wire approach and experiment to determine which is quietest in your environment.
If you're running a Decca all bets are off.