Can you use a My Cloud as a backup location for multiple computers? Yes.
However, do a forum search for Time Machine. Some are having problems (seems to be mostly El Capitan related) getting the Time Machine backup to work properly. Also there is a huge El Capitan thread that discusses a variety of issues with the My Cloud. There are some possible workarounds. See the following two threads/posts.
https://community.wd.com/t/updated-solution-to-osx-10-11-5-smb-error-36/165230/4
https://community.wd.com/t/permissions-access-issue/166703/2
Generally it is best to have the computer connected via Ethernet cable to a Gigabit router/switch to get the fastest backup speed. Trying to backup over WiFi typically is slower than wired Ethernet.
