You can use
window @win x y w h to move a window, and you could use the channel window's x,y,w (
$window(#chan).x, etc) to set the placement of your @win.
But if you want to move the @win....I think to do it purely with mIRC script you'd need to run a polled test (with a timer) to see if the #chan window had moved, and adjust the @win's at that time.
Alternatively there are probably some DLLs that will dock two windows together. Someone else would have to comment on that, though; I avoid DLLs where possible
