first off, gratz on the team and welcome to the forums.

This may be a dumb question, but you do have your main set as the focus target with your alt right?
other than that, the macro should work, sometimes it takes a second for the server to actually "assist" your first toon, so you can't hit assist and immediately start casting, it might take a second for the server to respond to your alt to tell it what to assist.
Make sense? lower ping times will definately affect this but there will always be a lag time between when you target something on your main and the alt picks it up