First thing is make sure user doing installation is domain admin and got access to both SQL server as database administrator
Also Which server you are referring when installing CRM 2011 system? are you referring virtual server (which configured to cluster SQL servers)?
Please refer following article which specifies three option to install CRM 2011 environment in cluster environment.
http://technet.microsoft.com/en-us/library/hh699768.aspx
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