Esta é a resposta encontrada em Re: vários problemas de DNS da WAN quando a primária falha
Edit the default LAN rule in Firewall->Rules by clicking edit on the rule that has a row that contains "LAN net". Then change Gateway setting drop-down to whatever you named the gateway you created with the Wan1 fallover to Wan2.
Here is a link to a simple and basic working guide for multiwan setup on pfSense 2.0. The top of the guide is for 1.2, but scroll about half-way down to see the 2.0 guide: http://skear.hubpages.com/hub/Dual-Wan-Router-How-To-Build-One-On-a-Budget. Combine that info with the other guides out there for setting up traffic shaping and it works great!