Dear N:
>
> You wrote:
> I have stopped eating meat for two weeks.
> I am not eating meat for two weeks.
>
> Can I use these sentences when the time is within the two-week period of not eating meat?
>
The first is present perfect. The action is complete with respect to the present. You have already been on your diet for two weeks. The answer to your question is no for #1.

The second could be spoken in the middle of the diet, though more commonly understood that the two weeks were yet to come. Progressive does suggest a continuing action, the adverbial qualifier throws it off a little. Context matters. The answer to your question is maybe for #2, but only in the right context.