Hi
At first poing i wouldnt go for bundling 2 links which are having different speed characterstics altogether (T1 - 1.54Mbps , ISDN - 128Kbps).
As a workaround if you are interested you can configure the isdn to fire out when your main T1 goes down,thats acting as backup interface to get rid of unavoidable disconnections/downtimes with the main T1 link..
regds