Between your /assist and /petattack lines, add the following:

/stopmacro [noharm]

There is a delay between when your main targets something and your other clients "notice" that your main has retargeted. /petattack, by default, will acquire a random target if you don't have one already. Adding the /stopmacro [noharm] to it prevents that from happening, so your alt will only acquire targets via assist.

I had the same problem. Very annoying, especially in instances.