With some OSA H/W it is impossible for two or more guests to
communicate with each other via the OSA card they have shared
between them. This is because there is no hardware shortcut such
that the card can microcode can sniff outgoing packets from one
guest & pass them over to the other guest.
The workaround for this is to make an ip route to the other machine
"route add <other_machines_addr> gw <gateway_addr>" and
do the same on the other machine. Alternatively in cases tested it
was found that configuring the OSA card as half duplex allowed the
guests to communicate with each other.