I don't see an attachment, however in a two (or more) interface setup you simply need to ensure that the appropriate static routes are in place. The gateway to the internet (or upstream device) should be the 172 link and that will be your default gateway. Then if there are no other networks other than 10.10.10.0 are on that interface the interface route will be sufficient for routing. If there are other networks that need to traverse back through that interface ensure you add a static route for those networks that point to the gateway of that connection.
If it is 10.10.10.0/24 and that is the only network then no extra static routes are needed, just the default route going out the gateway of the 172 network.. (Which is what I believe your post described)
Im able to test my objective in my virtual machine. Using 2 different subnet without adding a static route. Between 2 interfaces of the MWG will autmotaticall route or connect from 1st sto 2nd interface as long the 2nd interfaces (going out) able to reach the DNS