I think you are on the right track.
I have a similar setup on my home network uisng two dynamic IP addresses from my ISP.
Here is how I am setup:
Cable Modem from my Cable ISP (no built-in router).
Output from Modem connected to a 5-port Linksys Switch.
Port-1 of switch is connected to my 1st Linksys SPA2102-NA ATA with built-in NAT Router. This ATA is strictly configured as my primary VoIP service with my preferred BYOD VoIP carrier. This ATA is also configured for QOS to only allow ~ 50% up my ISP available uplink data speed.
Port-2 of switch is connected to my 2nd Linksys SPA2102-NA ATA with built-in NAT Router. This ATA is configured to connect to my secondary BYOD VoIP service provider. This ATA has its QOS configured to only allow ~ 50% of uplink speed availably from my ISP. Output of this ATA Router port is connected to my Linksys WRT-54G Router. My main PC is connected to this WRT-54G via Ethernet cable. My laptop is a Wireless connection to the WRT-54G.
Using two dynamic IP address assignments with a switch splitting service to two separate Linksys SPA2102-NA ATA's has worked really well for me and my BYOD (bring your own device) VoIP services.CQVoIPhttp://www.cqvoip.net