First, you will need to create separate global and nat statements for each inside group. Use 'nat 1 ..' to map one inside group to a 'global 1' outside address range for one ISP. Use 'nat 2' and 'global 2' to map the other users to the second ISP range. You have to be able to separate the hosts by subnet to do this.
Next, one of your outside routers will need to be able to do source address routing. With Cisco routers, this is done with route-maps and policy based routing. Configure it to send packets with a source address from one global pool to its ISP and from the other global pool the matching ISP.
This can be a major pain. If your ISP provides the routers, or they're small soho routers, it may not be possible at all. I found it easier to do the policy routing in my core router and send the traffic out two different firewalls. A little 501 or 506 can easily handle 5 users, are fairly cheap.
Greg
