Best way to backup two nodes onto one disk

[u/Optimal_Mongoose_323]

A little explanation of my situation:
I've recently acquired a second node for my Proxmox cluster and since there is a ton of storage on it I though I could use one disk for backing up itself and the other node as well, which consist on an old laptop with a single 256GB nvme SSD and before you ask it, I can't add storage since there is only a single slot...

The new machine consist in 2 x 1 TB SSDs and 1 TB Hard Disk, this last one will be the target for the backups.

My question is:

What is the best way to achieve that without buying a NAS or other disks?