With SQL cluster 2008, can we run two SQL servers simultaneously on the same database?
The real question was, is it possible to load balance query on multiple SQL server for the same database
I think you've got some of your terminology mixed up.
When you install SQL Server, you are generally installing an instance of the database engine. An instance of the database engine can contain many individual databases. An instance can be installed on a standalone server or a windows failover cluster.
You can install multiple instances of the database engine on a given server or failover cluster. In the cluster scenario, each instance can only be running on one node at a time. However, you can move all the instances to the same node if you want.
I think what you're trying to ask is whether you can run multiple database engine instances on a single cluster. The answer is yes. You can run as many as your hardware will support.
If I understand your clarified question, you're asking whether it's possible to have a single database mirrored across multiple different servers and then load balance queries across the mirrors. I think the answer is no. You can setup database mirroring easily enough. But I don't think there are any query load balancing features out of the box. There might be a third party product that can handle this though.
Depending on the database schema, this is possible. It is called Merge Replication. You can have changes made on both the publisher and subscriber databases and have them both in sync.
If the database schema wasn't designed with this in mind, it's very likely that you will run into problems with primary key duplication.