Quick answer:- when PPPoE server #1 hits 300 custys, I doubt there is a reliable way to make PPPoE server #2 start taking the remainder. You could probably do it with some tricko scripts however it'll be messy.
We've noticed after 200+ PPPoE users, MT sometimes becomes unstable.
PPPoE servers run happily in parrallel (or 6 running together in our case) and as a rule of thumb will 'generally' load balance themselves.
Rembember PPPoE is layer 2 and you *shouldnt* have an IP on the PPPoE server interface for security reasons.
For redundancy you simply run them all on the same VLAN or layer 2 segment. The first PPPoE Server to reply to a CPE PPPoE login request will get the job for the duration of that session as long as the serice name set in the CPE is blank. If that PPPoE Server (concentrator) dies, the clients connected to that PPPoE server will automatically broadcast for another server to login to, then the next 'first one' to reply will get the job....
***off the beaten track but helpful info***
If your CPE's are any good, the above process all happens very quickly, ie 1 or 2 secs. Cisco's, M0n0wall's, and *many* Linksys CPE's are good. (Believe it or not, our testing has proved that all versions of MT have the worst PPPoE client ever seen. It will NEVER login again by itself unless its rebooted or the watchdog forces it to reboot... :-)
M0n0wall has the most reliable/fastest PPPoE client seen to date.
Dont bother if your CPE's comprise of too many MT's !
Out of 6 x MT boxes running a PPPoE server in parrallel, 9 times out of 10 we see the same amount of customers logged into each box.
Hint:- dont forget you need to run a dynamic routing protocol upstream of your 'parrellel' PPPoE servers, or proxy arp if your feeling lazy :-)