How can I accomplish VPN across Dual Wan connections in a optimal way?

[![plan_for_network_desing_v2][1]][1]


Idea is to make:

 - Site to site VPN. 
 - **Make single connection inside the VPN to use full bandwidth of both wan connections** using (Split TCP, Multipath TCP, Bonding or technique x?). 
 - Make it possible for a Road Warrior to have VPN connection to both sites and have traffic use the optimal speed/route.

	


Note

 - The order for "split/bonding" and VPN might not be in optimal/working order in the plan_for_network_desing picture. 
 - The wan2 and    Road-warriors connections are 4G so they are behind ISP-NAT. So they can "only do" outbound IPv4 connections. Public IPv6-address might be possible. 
 - The DSL connections have public IPv4 addresses and take incoming connections.


I am interested to know how can I accomplish this?

 - What techniques you recommend and how should they be implemented?
 - What open source software/distributions would you recommend?
 
  [1]: https://i.sstatic.net/y6q37.png