If you are using pfSense 2.4.4 or later, automatic gateway switching (aka Multi-WAN) does work with DTTS enabled.
To configure, create your gateway group as you normally would. Then under System / Routing / Gateways set your "Default gateway IPv4" to the gateway group that you want to apply to users on DTTS.
What happens when the default gateway switches
When pfSense switches the gateway any existing IP passes will be erased, and devices on your network will need to re-query in order to be allowed to access the Internet again.
Keep this in mind when testing. You may have to use the dig or nslookup command to do a new DNS query before being able to ping a web site for example.
Load balancing vs. Multi-WAN
Currently load balancing your WAN is not supported with DNSthingy.