When my husband wanted to grind dungeons for the newest relic line for me when I was at work, we needed to buy a PC copy since I was on the PS4.
He also owned the PC copy cause that's where he plays.
We bought it on Steam though so we had to get the license (for me) and my steam account involved.

So yeah, two PC copies...and if you're on Steam, two different steam accounts as well.
I wish this game was more like FFXI where you buy it once and you can play it anywhere.