Are you wanting to provide you and your friend with an alternative connection if your primary one goes down?
Not that cheap to do unfortunately as you would both require too connections, one each to the outside world and one configured as tunnel between your two routers but as Matt says it would require a bit more than your average consumer router to do this.
If you both just want a backup connection for emergency use a simpler solution might be to buy a router such as a Billion 7800DXL which has the capability to use a a 3 or 4G dongle as a backup and can be configured to failover to this if the primary connection fails but this would require additional outlay on either credit if you use pay as you go or a contract.
Virgin (ADSL) => Namesco => Newnet => O2 => Plusnet => Zen => Newnet => Zen => Freeola => Vivaciti (using O2 Wholesale DSL) => Xilo (C&W Wholesale) => Xilo (O2 Wholesale) => Xilo (TT Wholesale due to O2 Wholesale closure)
Router: Billion 7800N
Note: I don't lay turf for anyone. astro or otherwise, all views and opinions expressed are my own based on experience.
Edited by techguy (Thu 06-Mar-14 18:56:48)