You can only have one default instance on a machine. An installation will create 0 or 1 instances, depending on what is selected. If an instance is installed, it can be default (if no default already exists) or a named instance
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass