Take another look at the original sentence:
You wanted to fix it this way:
When you have two main clauses joined with just a comma, you can fix the problem by subordinating one of them. Because is a subordinate conjunction that will do this job. When because comes after a main clause, however, you do not need a comma with it. An important rule to remember is that 99.99% of the time, there is no comma before because.
You might want to consult the rules for fixing comma splices and fused sentences.