Oh my god if I see this one more time Andraste help me fuuuuck. They're not "two sides of the same coin", they're people who think and see the world in completely different, if not outright opposite, ways.
For Fenris, what he experiences personally or sees with his own eyes is what is important and what is real; whatever theory you concoct is very nice and all, but this is what counts, this is what's solid - this reality we live in, the facts, what we can almost empirically prove to be true.
For Anders, what's important is the theory, the ideas, the moral principles, the logical consistency of something or the lack thereof. Current reality and the facts, statistics are almost trivial; what's important is what we can think of and, as long as you can't prove an idea's impossible, Anders sees it as the solid thing that's important, that the focus should be on.
Every single mage in Thedas could turn to demons and blood magic, and Anders would still go on about all the stuff he goes on about. Because the way he looks at the world is that it's the concepts that are truly real and solid, and the current facts that are flighty and changeable; for Fenris concepts are flighty, nothing but pointless daydreaming and navel-gazing, and the facts are what is real and solid and things to go by. This is such a fundamental difference in the way these two men even think;they could share all opinions and they still wouldn't be BFFs because the ways they got to those opinions would be so different and basically they'd still have nothing in common.
