Thanks for your detailed answer which will be an example for me (my English written isn't very good).
Your description is exactly the same as my trouble with my alt.
I tried isboxer where gathering was working for my main and my alt, but after a few tries, isboxer does not work anymore (it's difficult for me to obtain help about that because of my poor understanding of English).
So i decided to use Autohotkey which is working fine for my dualbox, except gathering.
I think that the solution is, as Jabberie says in his post, to tell to AHK to sync the cursor of my alt over the node, so his IWM should work