How to share a FUP between two seperate Internet services "Data accounts"

At the company that I work for we have a deal where a client can get a second internet service with us, and pay only 60% for the second line.

The Question is how can I share the FUP between theses two lines.

Hi Johan,
Did you ever work out how to do this? We do a similar service were the CAP can be shared between two connections