Typicall tech co-founder in a early startup person is the one who can code your whole application without any outside help. He/she should be able to go through the code and fix any bugs if it is needed. This person should have good knowledge in all aspects of technologies. For ex: if you are using a website built on .NET Framework with SQL Server backend, generally then this person should be familiar with databases, writing store procs, building web services, writing C# code and lot of other stuff.
When evaluating a tech co-founder you should ask with 2 of your guys can you complete your application end to end and go public without any outside help. If the answer is yes then he/she can be your tech cofounder.
Since in the early days without any funding, money is tight you should be able to complete the prototype/product with almost no resources.