No servant can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon. – Luke 16.13
Trying to serve two masters is tough. Serving one always gets in the way of serving the other and vice versa. At the end of the day you can only ever really have one master. You might be able to balance things for a while, but at some point you have to decide who you are going to follow.
I am a citizen of two countries. Fortunately these two countries are friends and allies and it is not a conflict to obey and submit to both authorities.
But let's say the unimaginable happened and the US and Ireland, as a part of a Europe went to war. If that were to happen and I were a younger man there is a chance that both armies would call on me to serve. In that case I certainly could not fight for both armies.
There is a pretty good chance that that is never going to happen. But that kind fo thing goes on every single day in my life and my flesh is in open conflict with my spirit. I cannot serve both of them. It is utterly impossible because they are at war. When we try to serve God but try to keep one foot in the world we are going to fail. When we try to serve the Spirit but keep walking after the flesh we are doomed to failure.
We must choose to serve Christ and Christ alone even as we live in this world with so many other potential masters trying to call us aside.