It really is up to you, but I wouldn't use them together. They're just too matchy.
I agree with the pp. I wouldn't use them because I find them too similar (the sounds you like from Riley probably make you like Rider, too), but if you come back to Rider again and again, maybe it's the one. As far as suggestions go, I think you could do an R or a long I sound, but not together and/or up front:
Miles & Milo
Silas or Cyrus
Too similar, sorry :(
Have you considered Strider/Stryder?
They are much too similar, imho.