I encountered this same issue and found a solution with help from this, and an additional pearl, but some background first.
You can add more than one Time Machine backup to the one that exists as a standard “Share”. I can’t recall what it was named out of the box because I changed it, something like “Timemachinebackup” or some such.
If you select this from computer #1 as a backup and do nothing, it will expand out of control to take over your hard disks because Time Machine is designed to keep old versions, deleted files etc. until the disk is utterly FULL. This is fine with a 3 TB external USB drive perhaps, but not with a 20 TB RAID 1 NAS. As the previous responder points out, you can go to Settings…General…Mac Backups and click the little “configure >>” in blue. There you will find a slider that can be used to limit the maximum capacity for your Time Machine. I personally set mine to 50% more than the computer to be backed up to allow plenty of room for recovery of older files which is the whole point of Time Machine.
You CAN add the same, one and only My Cloud Time Machine Share as a destination disk for the Time Machine on a second computer, but the trouble is, I do not see that there’s a way to independently adjust how much space is allocated to each computer.
Therefore, create a new share with a different name. According to JoeTaxpayer above, you need to “bless” it with the “Settings…General…Mac Backups…configure >>” process which I did, I bumped the maximum capacity allocation to the left a bit to perhaps trigger the My Cloud to recognize it as a valid Time Machine destination. I don’t know that you need to go bless it in this way to make it appear as a selection on your second computer, I don’t know if you have to bump the slider to make this happen, but it did work and I won’t be playing around with it to see what the minimum requirement is!
But I can tell you this. If you rename the existing TimeMachine share to something else, create the second share with a different name as above, and then go to start the Time Machine on computer #1 it will only see the second share you created. You have to actually start the backup in process for computer #1 before creating the second share, and then, and only then, will the second computer see that share.
After you are done creating the first backups, don’t forget to go back and change the allocation of data to what you want if you didn’t before, lest the Time Machine hog all space on the NAS.
Whew!
Could have been better designed, if you ask me.
