I voted "too matchy" but I probably should have voted "other" because the problem for me is that they're too theme-y. Luther is only one letter away from Uther who was King Arthur's father in the legends. Just a heads-up- while I wouldn't do it myself, perhaps you were trying to do a theme. That being said, I love both names individually and would love the theme on a sibset whose parents were historians/classicists/etc.
I wasn't aware of Uther, lol. Good to know though.
I think they are a little to close to use, but like the idea of Arthur as a mn instead. Other fn ideas you might like:
I find them a little heavy to pronounce together. They are great names though. I really like Magnus!
Sorry I think they are a bit too matchy for brothers but that is just my opinion and I think it could work if you liked it enough.