I've found the solution:
-installed WMWiFiRouter on Diamond
-rooted and added ad-hoc patch on the A100
i've rooted with this guide
and added ad-hoc with this one:
batista70phone.com/2011/05/supporto-per-reti-ad-hoc-su-acer-iconia-a500/ (is in italian, if you need english tell me, I can translate it)
To avoid problems open the page from the tablet and copy/paste the commands, and remeber to SWITCH OFF WIFI AS SOON AS POSSIBLE before playing with the wifi files!!
Everything works. You have only to start WMWiFiRouter on diamond and than looking in the wifi list on tablet for the wireless just created ( Office Lan....etc ).
For convenience activate the "wifi always on" also if "display is off" trough Diamond Tweaks.
I hope this can be useful for someone else ;)