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You don't use link-local addresses in this way. The point of the link-local address is that it can always be directly inferred by it's MAC address and not something you would typically manually assign. There's also annoying specifics with link-local addresses that make them less usable, specifically that they MUST have an interface identifier on them when entering them in any software, like accessing a webpage by link-local address. For example, you HAVE TO do http://[fe80:🔢56ab:cdef%eth0] you can't just do it plain, as the link-local address is meaningless without the device identifier.

The IPv6 link-local fe80::/64 range is like IPv4's 169.254.0.0/16 range, except it doesn't pull numbers randomly out it's as, and again: has the additional oddity of requiring an interface ID at the end. Meanwhile that creates an odd predicament with DNS, even with a HOSTS file.

Nonetheless, what you're looking for instead is the Unique-Local addressing, using the fc00::/7 range, which is more equivalent to IPv4's 10.0.0.0/8 and other private-use IP ranges. Within fc00::/7 it's recommended you only use fd00::/8

If it's two computers directly wired together, you could pretty much just number them as fd00::1/64 and fd00::2/64, or whatever values you want.