In general, Appletalk should work
find over MPPP - in fact, there isn't
much of a difference with Apple over
PPP as over MPPP. If you see routes
in the routing table, pings work between
the routers, but further pings do not
work, I would suspect the the Apple
portion of the network and not the MPPP.
To confirm, you could disable MPPP, and
see if you can ping with just one
T1 up. If you can, then you will have
to troubleshoot the apple portion
of your network.