Question regarding using 2 (or more) ATAs at a single location.

I currently have 2 ATAs setup so I can have 4 different lines. Works fine most of the time. But every now and then we have an issue with inbound calls not ringing through. So I figured I should find out what is the "best practice" for this scenario.

Should the 2 ATAs be on the same subnet? Or cascaded (one plugged into the other)?

When forwarding ports through the ISP's (Comcast) firewall, I would usually pass 5004-65000. But what should I do in the case of 2 (or theoretically more) ATAs? Is it enough to just pass port 5060, 5061 (etc) to the specific ATA?

Anyone know the most stable way to configure this?

I wish I knew more about what all ports were doing what.