Pick a large ADSL company and the contention will not really be an issue as most people on average dont use their adsl for more than a few minutes a day :P
Dual bonded ISDN is a crazy idea, you need a 128kbit stream upload and your going to do that with exactly 128Kbps max upload, i think not, you wouldnt even be able to fill the buffer as you cant go fast enough
I Never have had a problem sustaining >128kbps on ADSL with NDO or Eclipse.