It's more of a warehouse job than a science job, so I'm probably not qualified to help, but I love learning, so I did some reading.

Different mixes of CO² and nitrogen are available for both carbonating/nitrogenizing beer, and further mixes designed to pressurize the lines for dispensing. Replacement beverage o-rings seem to come in a number of materials from polyurethane, silicone, teflon, and others and looking at o-ring compatibility charts, they all seem to both be listed as compatible for nitrogen and CO².

Since you're not dealing with liquid gas, I don't think you need to worry so much about material as if you're using something food safe made for beverages, it doesn't seem to be an issue what they're made of or which gas you use as far as I can find. You also shouldn't need to worry about the nitrogen freezing the CO² and forming dry ice from the amount I could imagine you using at home.

Without knowing more about what exactly you're working on, that's the best general help I can dig up. Depending on what exactly you're doing, finding a good homebrew or scuba shop/forum could probably get you the most reliable answer to what you're working on since they'll both be blending those gases in a manner safe for the human body.

I hope that was at least marginally helpful!