Let’s say that you do a multiple spirit evocation of an angel and a demon; would they fight or not be friendly with each other, or would they be chill with each other, and even work on the same task you give them?
For example, let’s say you evoke a demon and angel for baneful purposes; would they disrupt each other, or would they both work on the task you request them to do?
Angels and demons often work together just fine. It’s a common misconception that they don’t because Bible.
Though I’d ask them both if it’s okay first, because the demon might prefer to work with a certain angel on these things, and vice versa.
Angels and demons are, in a sense, two sides of the same coin: they complement each other more than oppose each other but you will experience the consequences of your perspective regardless. It is entirely about states of mind. If you drop attachment to the stereotypical angel definitions, you could have nasty experiences with them ( 
but usually we don’t because we tend to like and get attached to these definitions ) and, likewise, dropping demonic definitions could produce very positive experiences with them. From the perspective of dual/illusory experience you cannot experience the illusion of choice without contrast, i.e. you must have the poles. So-called “Balance” is about recognising the utility of both in the dual experience.
Like others have said, it’s a myth. Once I accepted that it is, I started calling upon them both daily and I feel quite balanced in Life.
I intend to dedicate a day of the week to my Patron Angel and another to my Patron Demon. By the looks of it, this will be Samael and Astarte or Lucifer.
Work with both but DO NOT have a negative mindset, think of the energies as day and night.
Never forget that they are all man made none of them would exist if no one believed in them. They need you to exist.
I like how of all the angels your Guardian Archangel is the most demon-like angel in the choirs of heaven.
I admire him because he’s misunderstood. He was Demonized (and in turn became the Ruler of a Qlipa and had one named after him).
That being said, he also represents necessary destruction, Karmic punishment and freedom of choice.