I am buying keepsafe collars for my two.
I read a terrible story on some other forums.
A girl on there came home to find one of her dogs dead.
Her other dog's jaw had gotten stuck in the collar during play, causing strangulation. The other dog very badly bruised its jaw also.
Keepsafe collars come apart once pressure is applied to them, but have double 'D' loops which you attach your lead to while walking.
KeepSafeCollar.com break-away collars for your dog
My two will never be left alone with a plain flat collar on again.