IT WORKS !!!
I don't know the WHY details, but it works.
what I did ?
First, I upgraded to 1.2.3 release nanobsd on both sides.
Since there was messages in the log saying there was an error trying to add the routes in my custom options, I tryed first to remove all routes in custom options to see what append ! The result is that it works without any custom option anywhere!
From both sides, I can take control of PC on other side (ultravnc) by using their respective IP addresses (192.168.0.* or 192.168.1.*)
And now I have to do the bridging stuff, just waiting for the tutorial to be updated.
PS: By the way, I discovered that the firewall in windows XP SP3 prevents the PC to respond to pings if activated. silly thing :)
Patrice