Fix Duration vs Assign Multi Resource ??

Hello everybody!!!!

I am a new member and new in planning field...Please help/teach me...;-)

I want to maintain/fix the duration and at the same time assign multi resource but the problem is when i put extra resource, the duration will decrease! and opositely will happen if i decrease the resource!!..
I try to rid off effort driven..and do some try and error..but the problem still occure.What happend actually?
Is there any way to fix the duration?

Thank you

regards,
Zureen