I want to connect to two different ISPs at a time

[Jason]

I have two routers, one to connect to a special service ISP, through ISDN connection, the other for general Internet use. I would like to activate automatically one or the other depending on the application calling the appropriate public IP. I need only to automatically connect the special ISP IP when calling it, and this must be through the special ISDN router.

Now I only can change the gateway setting every time, when I need to switch.
Is there any software router utility that can recognize the IP Address and route the request through the special router?

Hope it is clear enough ...

[Mel]

It's very simple to do if you can clearly define the circumstance that should invoke the ISDN router.

If that circumstance involves a known set of destination IPs, that is even easier. You would simply enter in a static route on the Internet router to point ISDN router.

If the criteria is based on the source IP or on a service such as Citrix or some custom TCP or UDP port, then you need to use "Policy based routing" to achieve the same thing.