If a person fasts for more than one concern at the same time, does it dilute or diminish the power or help that could be available were we to fast for any one of those concerns individually?
I suspect the problem here is one of viewpoint. The fast is not a magical rite that invokes a quantity of blessings on the object of the fast, with the consequence that those blessings might be diluted if spread too thin. We are not trying to bribe God or secure his divine favor through His appreciation of our performance of a fast.
The fast (and the prayer that should always accompany it) is a blessing that God has given to us. It is completely for our own benefit. The fast is designed to increase and deepen our own spiritual sensitivity, allowing us more intimate communication with God. In that state, we can more easily determine God’s will and understand how our own actions might affect the course of events. We become more sensitive to impressions and ideas that can help us and those we love.
So why not just fast for everything at once, all the problems in our lives and in the world? Because that is not how our minds and our spirits work. We normally confront our challenges individually and deal with things one by one. Eveything at once is simply too big an elephant to eat in one bite. But as we consider individual challenges and obstacles, fasting can help us understand how to proceed, or at times give us the strength to carry on despite the challenges.