Currently, the way maas deals with multiple interfaces is to use policy routing for any gateways that get defined, per interface.
The difficulty arises if the customer wants control over which gateway is used as the default.
Current way to do this is by unsetting subnet gateways, and defining a single gateway, per machine, via cli:
maas admin interface set-default-gateway $machine id $link_id $gateway
At a minimum would like to be able to set a gateway, per machine, via the ui, and preferably for the way the ui deals with gateways to be improved, overall