What you are talking about requires source routing, and is impossible per se.

However, another option would be to use a proxy server arrangement on the  
other end of the second ppp link.  Point your internal hosts at that proxy  
server, and traffic is routed to the proxy server address and then from there  
on to the destination, depending on what the routes at the end of the 2nd link  
look like.

Traffic originating from your ppp hub (the one with the two ppp links) would  
be routed to whatever the routing table says for a particular destination.

Nonetheless, this requires that the service you are using can a) be proxied  
and b) proxy software is available on the internal network, which may or may  
not rule this out as a solution for your problem.

However, I would recommend finding another solution for your network design.   
Whichever way you turn this, it is pretty sick.  Even if you can do source  
routing, I would strongly advise against it.  It is bad design(TM).

If you want to take this off-line, I will gladly see if we can come up with  
something that fits your needs better, since this goes beyond the scope of  
current-users, IMHO.

Best regards,
--
Christian Kuhtz <ckuhtz@paranet.com>
                                                          ".com is a mistake."