Lacie Thunderbolt 6Tb Hard drive dual thunderbolt connectivity issues

I have two desktop iMacs, that are connect each with a thunderbolt cable to a 6Tb lace Hard drive, only one of the Macs seem to register the hard drive at any one time. I was under the impression that I would be able to simultaneously use the hard drive from each of the macs independently of each other. This is frustrating, because to access the hard drive, I have to turn off one of the macs, to then access the drive, this can't be right, is there an app I can use?😕