Mavlink connection

Hello. I have a problem - i need to conect 2 raspberry and pixhawk in 1 net Mavlink, but the sysytem must be fc->rp1<- ->rp2 because i havent free serial port. Could you help how comand in mavproxy or how to setup rp to do this? I didnt found any videos or gyide for this and i dont know are this posible