I hate to the the nay sayer in all this discussion. Are you aware that 2 dogs playing, can get tangled up and get caught in the collars. I just wouldn't want collars on 2 dogs unless I was right there.
I have 2 stories about collars getting caught on teeth. One, 2 golden puppies were playing in the water, and got their teeth caught in the collars. Both drowned.
One, a 60 pit mix, got her collar tag caught on a metal floor heat register (house was 100+ yrs old and had heavy registers) Fortunately I was right there, and told her 'down' and got her collar off.