I played rdr2 first and later played rdr1. Playing 1 made me want to play 2 again, which I did and loved it even more the 2nd time rpund.
I personally don’t think playing them out of order ruins anything story wise. It’s like watching or playing a sequel as opposed to a prequel if you play 2 first.
But…whilst rdr1 still looks and plays great, playing 2 first may take some of that away, as you’ll be used to rdr2 which is just about as perfect a video game one can be. I’d therefore recommend 1 then 2.