For a horse that has very dry hocks and stifles, with good X-rays In between injections. I give adequan loading dose 2x a year. Would pentosan in between be helpful as well? How much, how often?  Weekly, bi monthly or monthly?

Feeding Exceed 6 way, just started equithrive as well.

In my opinion you are way over supplementing. I would pick adequan or pentosan and stick with one or the other , not both. Your already using exceed 6 way , that covers a lot of bases. I would not use it and the equithrive. Pick one. 

I figured I was. Was already on exceed 6way but vet thought equithrive would be better because of the reservitol? 
I have heard that pentosan is better for horses that have existing problems and adequan for prevention. 

You are definitely over supplementing.  I would use Equithrive and Pentason.