i would just be respectful towards him as i would anyone else,he may be a Dom but he's not your Dom and anyway Doms are regular people too arnt they ...tho i do sometimes wonder lol

if you're a little unsure why not ask your Master to discuss this with the other Dom and see which they or most importantly your Master feels is most suitable? that way you dont have to worry,you can be relaxed and enjoy your evening and you know you've done the right thing and the icing on the cake is you will also have represented your Master very well