Universally unique 64 bit long generator
I would just use SecureRandom#nextLong(). SecureRandom will seed itself and shouldn't be the same between two similar computers at the same time. Also, it uses more bits of state than Random (128 vs 48).

I don't know exactly what computers you'll be running this on, but if it's something like a cluster of servers they'll probably have the exact same s1/s2/s3/s4. If they happen to start at the same time (plausible if two processes on two cores are launched by a parents at the same time), then that only leaves the 48 bits of random state.

If you use SecureRandom, you can just call it and immediately get a 64 bit UUID that will have as few collisions as you can possibly guarantee with two offline computers.