Unless one box can terminate both the ISDN and Cable connex,
you need a third box to make routing decisions for your lan.
NB That third box can be contained within one of the other
two. But for sake of discussion, its best to think of
it as a separate entity.
Now in the traditional dedicated world, one would multi-home
via leased lines, and run BGP to manage the routes. This
also has the advantage of sending packets to the provider
that is better able to deliver them. And if either line
goes down, the BGP session shuts down, and those routes
are withdrawn from your routers table.
I rather think it unlikely that a cable ISP or a dial-up
ISDN ISP will offer BGP. First off you have to have your
own AS and you have to have you own address block.
Unfortunately the shortage of available network blocks
means that many of the "appropriate" solutions to everyday
problems are not available to everyday folks.
Again, one of your routers may be smart enough to do this
on its own, but I am going to talk as if you had a third box.
What you do is have a router talking to both boxes. Here,
a UNIX box makes sense for the router, it can be an old 486
with 2 ether cards, for it will need to run some scripts that
check on the outgoing connections, and tickle the appropriate
outbound router. The third box will act as a router to your
LAN, and will have separate views of the two outgoing boxes.
The script will periodically test the primary line, and, if
it is down for more than some specified time, it will send
packets to the backup router.
Assuming that you can't get routing from two such providers
to an address block that you own, the third box will need
to run either NAT or DHCP, so that the operational results
are appropriately reflected to your lan.