so to clarify, have your main (click) button have the following macro:

/Click MultibarRightButton1,,,,
/Click ,MultibarRightButton2,,,
/click ,,MultibarRightButton3,,
/click ,,,MultibarRightButton4,
etc....

Substitute "MultibarRightButton1" for another button location where you're putting your castsequences
To find out what a button is called use this macro:

/run DEFAULT_CHAT_FRAME:AddMessage(GetMouseFocus():GetN ame())

now your MultibarRightButton1 will have:

/cast [@party1] renew

MultibarRightButton2 will have:

/cast [@party2] renew

And so on