In my experience (GA where it happens a lot) it's quite common for ATCOs to respond to a a situation where one aircraft transmits over another's transmission by prefixing his response with with 'two together'.
In GA I also find that the term 'stepped on' is used most often by pilots to describe when an ATC message gets transmitted over by someone else. Useful from a safety perspective, IMHO. Agree with the Welshman, 'blocked' probably better as less syllables and it should be in CAP413 (mine's out of date so it might be now).