Aggregating 2 ISDN USB TA

Hi all,
I recently have 2 ISDN lines and I'd like to join them as described
here:

http://lartc.org/howto/lartc.rpdb.multiple-links.html

I'm not quite expert so I'd need some advices.
I tried it on a Ubuntu 5.X.

1) The TAs are autodetected and available under /dev/ttyACM0 and 1 but
the first problem I ran into is "how do I create a ppp1 interface"? I
googled around but I could't find it. The Ubuntu guide says to press
"ADD" under connections, but the button's not there . Anyway I think
the CLI approach would be the best one. The ppp0 oone works ok.

2) in http://lartc.org/howto/lartc.rpdb.multiple-links.html what does
it exactly mean by

"let $P1_NET be the IP network $P1 is in, and $P2_NET the IP network
$P2 is in"

Would it mean 255.255.255.255?

Knowing at least these matters would let me do some tests on the field,
thanks in advance for the help!

--
Poochie©