share one site-to-site vpn across subs

Hi There,

Quick one.

I have 2 separate subscriptions on my tenancy. Lets say Sub1 and Sub2.

Sub1(10.67 subnet) has a site-to-site vpn back to on Prem(10.64).

Sub2(10.69 subnet) also needs a site-to-site vpn back to on Prem(10.64)

Ran through this (using ARM) https://azure.microsoft.com/en-us/documentation/articles/vpn-gateway-vnet-vnet-rm-ps/

Created a vnet-to-vnet connection so now both subs (Sub1 and Sub2) can see each other internally.

How can I get Sub2 (which doesn't have a site-to-site vpn)  to route local traffic through Sub1 and back to on Prem and vise versa?