This is presently not possible with the ASA.

You can accomplish this with PBR upstream on a router.

Two methods to load balance with two ISPs upstream.

1. Add two globals one for each ISP.

2. Translate 1/2 the inside networks to one ISP and the other 1/2 to the second ISP global

3. upstream on a layer 3 device do PBR based on source IP from the fireall.

4. Send globals ISP1 via one link

5. send global ISP2 via the other link

second method:

1. you can use just one global.

2. and load balance between the two links to the ISP on the router.

-KS