Win10 share restriction via credentials

First of all, I realize this is a Win7 forum and this question relates to Win10, but I can't get registration to work on the tenforums site, so I have to post this here.

Is there any way to share a directory between two Windows 10 computers so the remote computer has to provide credentials every time it accesses it? I'm trying to set up an automated backup solution where a third-party software (narrowed down to either Duplicati, Areca, SyncBack, or possibly Syncthing) will sync the contents of the computer it's on to another computer, and that one will do the same, so they both contain a copy of each others' files. However, what I'd really like to do is limit the access that the computers have to their backed up data, allowing it only by the backup software. This way, if either computer is infected with malware, especially ransomware, it won't be able to affect the backup on the other computer.

It seems to me the best way to do this would be to require credentials (username and password) to access the share, and have them saved in the backup software, but so far I've not had any luck getting it to work. I've tried different variations of settings, and it either doesn't ask for credentials at all and simply blocks access, it asks and still blocks access, or it asks once, allows access, then doesn't ask again until rebooting (or so it appears). Is there a way to do this either natively or with third-party software? Otherwise, is there a better solution? I was thinking of blocking access to the remote computer to all but the backup software via firewall, but that seems like overkill and shouldn't be necessary, and I'm not even sure if it would work as expected.

By the way, I'm testing in VirtualBox, in case that matters. It shouldn't, since as far as Windows (networking) is concerned it's two separate computers on the same network that are connected to each other, but wanted to mention it just in case.

The simple way to do this is use OneDrive or Google sync with Google drive. With this you don't need shares and it will back instantly to the cloud it's all done automatically so no need for any other software

I'm setting up a local backup, not cloud-based.

It backs up to all pc that sign in with the same onedrive or google account

I could be wrong, and please correct me if I am, but I assume it does so via the internet, not over the LAN. Aside from that, I simply prefer not to use MS and Google products any more than necessary, especially cloud ones. While the intent is to encrypt the backups, I'd like an option that doesn't necessarily require doing so, which rules these out. It should be very simple to just do so over the LAN, as it's already all set up, and shouldn't require extra steps/software. However, if it does, I'd prefer a third-party solution to one provided by MS or Google.