Hello Richard,
do you only want to bring both links up so you can use the full 128Kbps, or do you actually need two different users to be logged in at the same time ?
If you need both B-Channels to come up, try to configure ppp multilink on your BRI or dialer interface and use the command ´dialer load-threshold 1´ or ´ppp multilink links minimum 2´ in order to bring up both links immediately.
HTH,
GP