Not advising you on a cover letter, just on the grammar...
You would want to say something like this:
I worked at Delicious Cakes for more than two years, starting in February 2012.
since would not work there very well, unless you punctuated the sentence like this:
I worked at Delicious Cakes for more than two years—since February 2012.
But there, since February 2012 is disjunct, and there is an ellipsis:
... — [I worked there] since February of 2012.
In a different structure with more complicated tenses you could say:
By July of 2014 I had been working at Delicious Cakes for more than two years.
That would be like telling a story, not simply giving the raw facts of your employment:
By July of 2014 I had been working at Delicious Cakes for more than two years, and felt it was a good time to try something different.