Not sure what your getting at exactly.
Are you talking about having an internal network as one “ISP” and having a pass-through to the public “ISP”? This can be done.
Getting two ISPs into a router, I am not sure would be a wise choice, as you could simply purchase another router, automatically and physically segregating the networks.
Hope this helps!
Schmidtw has obviously not come across this before. It is fairly common practice in larger companies. You have a connection to two different ISPs, to the same or different routers, and the traffic can come in or out of either ISP link, and everything works.
The only way I know to do this is to use BGP and peer with both ISPs. Then you can load share between them. You need to speak to both ISPs and agree to do this. It is not a simple solution to implement, and I would recommend that you need to hire someone to do the config if you don’t already know how to do this. If all goes well, and the guys at each ISP know what they are doing, it is not too bad, but if you hit problems, then hiring someone will save you a lot of heartache.