You recover the Gamertag to your retail console, then host a modded match on the Jtag (which is offline) and use a Ethernet cable to connect them both and get the retail console to join the jtag's game..
But then how do u get the first one to go online, bcause they only hav one ethernet port, and if u use it to system link u cant cnect to internet, so do u hav to use wirelessa/
you use a router. you plug one ethernet cable from your jtag and one ethernet cable from your retail both into the router so they are on the same network, but you make sure there are no live accounts on your jtag so it doesnt sign into live.