See inline.

HTH,
Best Regards,

Uttam Parui
Microsoft Corporation

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Hi all
I have Sql Server 2k configured in active/passive Cluster on Win2k3. Now
because the passive is just ideal and sitting there doing nothing, i want to
use it to host One instance of the Sql Server,
1> Can i do that, i mean Node1 hosting SqlSrv1 and Node2(passive for
SqlSrv1) hosting SqlSrv2 that is also live.

<Uttam> Yes, you can host another SQL Server. SQL Server 2000 Failover Clustering supports up to 16 instances of SQL Server (1 default, 15 named or 16 named)

With instances, come two new concepts for failover clustering:
Single instance cluster: replaces an active/passive cluster. A single instance cluster means there is one SQL Server 2000 virtual server installed.
Multiple instance cluster: replaces an active/active cluster. A multiple instance cluster is one in which there is more than one SQL Server 2000 virtual server installed. Because of the way the implementation of
clustering is different with SQL Server 2000, using the active/active terminology does not really apply.
Note that you can have only one default instance of SQL Server 2000. So if the first instance is a default instance then the second instance will need to be a named instance.

</Uttam>

2> If so how can i do that.. is there a white paper to do so or some
guidelines instructions on how to set it up.

<Uttam> You install the second instance of SQL Server in the same way as you installed the first one.

Note: Each SQL Server 2000 virtual server requires its own disk resources that cannot be used by other instances. These disk resources are the logical drive names (for example, drive F:\) used by SQL Server
on which to store data and log files. Separate physical disk sets are needed to make up the logical drive, unless your disk subsystem supports multiple logical drives on one physical drive set.

You will also need unique SQL IP Address and SQL Network Name (i.e virtual name) (and a name for the instance in case of named instance) for the second SQL Server.

For additional information, refer to

TechNet Support WebCast: How to install a Microsoft SQL Server 2000 virtual server on a Microsoft Windows Server 2003 cluster
Discusses how to install Microsoft SQL Server 2000 clustering on Microsoft Windows Server 2003 and differences between Microsoft Windows 2000 Server and Windows Server 2003.
http://support.microsoft.com/default.aspx?scid=kb;en-us;888121

</Uttam>

thanks