MS SQL Server instance name Quick question guys!

Will there be any issues if I use the default instance (MSSQLSERVER) for both test and production environments? They are on separate physical servers. I'm using MS SQL Server 2012 which will run on clustered Windows Server 2012?

Thanks in advance and Happy New Year.
That will be no problem to have multiple default instances in your environment. The only real restriction here is you can only have one default instance per machine. 

You will have different network names for the different failover cluster instances. In short, if I'm understanding your question correctly then no that'll be no problem.