Best practice advice?

Hi - I have two networked desktop PCs, which are left switched on at all times.

My backup strategy is to backup each PC to a disk on the other PC, on a nightly basis. However, having read about the possibility of malware that will encrypt all files on your PC, I am worried that this could also encrypt my backup files on the networked PC.

Does anyone have any advice on the best way to protect against this possibility?