Question : How to let two DHCP server co-exist on one sub-net and limit IP leasing time?

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Answer : How to let two DHCP server co-exist on one sub-net and limit IP leasing time?

You can't have 2 DHCP servers on the same network segment whithout getting the problems you describe. DHCP doesn't work the way that a user log on to it. For more information about how DHCP works, take a look at these NT Magazine articles:
http://www.ntmag.com/magazine/Topics.cfm?TopicID=119&Topic=Dynamic%20Host%20Configuration%20Protocol%20%28DHCP%29&Action=Articles
It will also describe what lease time is.
To summerize: What you are trying to do, is not the proper way of using DHCP and WILL cause you troubles.