When you say secondary outside connection. Did you get another range of IP's.
If so, It will be a great challenge. As far as I understand the PIX can only have one default gateway. I'm assuming your current default gateway points to the existing internet connection. So any traffic going to the PIX through the new connection will want to return through the first connection. Assuming you can get it to work, this will defeat your efforts.
I am also assuming that you have tested and gotten the VPN to work before ordering a new line. If not get the VPN to work first then figure out how to off load the traffic.
You might also be able to use two interfaces as outside interfaces. I have never done this and it probably is more of a headache to implement and support and not recommended.
Two solutions off the top are to implement BGP with the two providers if you are in a contract you cannot get out of.
Or just up the bandwidth from your current provider.