load balanced Netscreen 204 | 2 ISP

Hello all

I have a big question for my knowegde. 

I have a private network /16 on the trust zone. and on the untrust I have the ISP1 with public IP, 

I have a policy to make nat the private network  on the untrust interfaces.

but we are getting a new ISP, the second.

so we like to use like this:

use the nat or policy to make nat from trust to internet the both ISP, 50% ISP1 and 50% ISP2, and when the ISP1 it is come down all traffic come to merge to ISP2.

can I use the netscreen 204 for this and how?

Thank you