You actually need a headphone that can mix the 2. PS3 outputs audio through either a optical wire, or HDMI/RWY wires. It outputs chat via bluetooth or USB. As you can tell, they don't share a common output so to get them both to the same audio solution, you need a headphone that plugs into 1 output from each.
Getting them to mix in your headphone without some sort of ridiculous overlay or under tone is where a mixer of some sort would come into play. I personally have chat (from USB) / audio (from optical) going to an Astro Mixamp, which you can then plug in almost ANY headphone/mic combination into and it will mix game audio and chat seamlessly...giving you the options to either have 100% chat or 100% audio, 50/50, or any combination in between.