Exiting from one interface entering from the other one

Hi all,
I would like to know if it is possible (and if so please guide me step by
step) to send UDP packets (RTP in particular) from a specific interface
directed to another one of the same PC without passing from the internal
loopback path.
I have a PC connected to 2 subnets and I would like to sniff with ethereal
the packets outgoing and coming back to compute without needs of
synchrization in this way the end to end delay of this external path (by
considering the sending and receiving time of the same RTP packets.

Thanks in advance
Regards
Rocco