Music Law book, as we both agree on our terms? If the solo artist and the band operate as two separate entities, you should sign two separate contracts. That will allow you as a manager to keep separate accounts reflecting income and expenses. Otherwise, income destined for the band may be hijacked to pay off the solo artist's debts. If they are not separate entities -- for example, the solo artist hires the band as contractors and pays them an hourly rate -- then one management contract with the solo artist should suffice and there is little need to sign the band.
What about changing the agreement? It's more common to sign a management contract based on years (or financial goals), not on number of albums. (It sounds like you are crossing a record contract with a management agreement.) After all, what happens if the artist doesn't have (or keep) a label deal? Does that mean the management agreement ends? In any case, you're free to make any changes you wish to the agreement but we would suggest that when modifying the standard agreement, always review the instructions that accompany the agreement. Also, beware that in some cases, managers may need to be licensed in California.