@mvalpreda that is exactly correct, as long as the MX on site 1 can see the internet on WAN1 and the MX on site 2 can also see the internet, they will form a connection (A). If MX1s WAN2 can see MX2s WAN2 then they will form a connection (B).
You can then set whether A or B are used as a failover pair, or are load balanced (from each end for outbound traffic).
As the internet and the Metro-Ethernet have no direct knowledge of each other then you also end up being able to choose the port the traffic comes in on (by setting where it goes out of at the other end).
Remember to set the bandwidth on each WAN port (both up and down if different) if you are using load balancing as it uses those values.
You can also set traffic class rules to, for example, send VoIP traffic over the lowest loss / jitter link and file server traffic to prefer the link over the internet.