Andy, there's nothing at all wrong with running both Deca and EQ together.

For me, both Deca and EQ, are both long esters which could possibly have an effect on the transition to PCT. Second, due to their similarity, both Deca and EQ, work through androgen receptor activation.

This is where personal choice comes in, I would rather stack another steroid that exerts it's effect via a different pathway (i.e. Glucocortcoid antagonism) -
Glucocorticoid antagonism by exercise a... [Med Sci Sports Exerc. 1990] - PubMed - NCBI

My goal, when coming off a cycle, would be to make the transition to PCT easier, prevent post-cycle sides (e.g. gyno, low lbido, etc), and keep as much of my gains as I can while trying to minimize the time it takes to recover and return to normal.

Many people stack both together, and get good results. I would make sure to add some sort of test in the cycle as well. If taking Deca, and EQ, together for a cycle works for you, then by all means go for it.

So, it's just my personal preference not to run both together, but that said; it's my personal preference only and not based on some concrete evidence that it would be harmful to run both at the same time.