You can use the solution I mentioned on m last post (Chronobis) or this workaround provided by Chris de Herrera:
As of Activesync 4.x, for some unfathomable reason, MSFT has removed
the possibility to sync one device with two exchange servers, or two
PC’s connected to excahnge servers. This drive me insane when I bought
my first WM5 device. But after much googleing, I found a workaround.
So, to sync two exchange servers, put this in the device’s registry:
= (DWORD) 1
This key will most probably be there already, but if it is, its value
will be 0. If so, change it to 1.
Soft reset, then, setup activesync ON THE DEVICE (If you try setting up
on the PC, it will still fail).