According to Ignatios Souvatzis:
> 
> Our (or any BSD) kernel doesn't support multiple equal routes in-kernel, as
> far as I understand. You want to remove the ppp0 route to make it find the ne1
> one.
> 
In other words, (a) I should use a numbered ppp link (pity!) and (b) 
the -ifp and -ifa qualifier to route(8) do not have the desired 
additional effect of differentiating between otherwise identical 
specification.

I may of course be reading the rather vague man page badly:

     In a change or add command where the destination and gateway are not suf-
     ficient to specify the route (as in the ISO case where several interfaces
     may have the same address), the -ifp or -ifa modifiers may be used to de-
     termine the interface or interface address.

... and I still don't understand what argument I should give to -ifp.  
I'll be pleased to help making the above clearer, if someone tells me 
what it really means :-)

++L