My book says conditional clearances may only relate to one aircraft
HD - Not sure that the rules have changed, maybe the LL guys are using a creative meaning to this.
If the clearance is worded as " After the 3rd BA 320, line up and wait" you could argue that the clearance relates to only one a/c. The fact that the 3rd Ba 320 is on a conditional subject to the 2nd, and the 2nd subject the first, an argument could be made to say that each conditional relates to 1 a/c ( I'm not sure i explained that very well but hope you can get my drift!!)
Personally, i tend to try and make a judgement call based on what I think the crew can understand I am trying to tell them. If i think the pilot can't speak good english, or, I am not explaining myself well ( 9 times out of 10 I'm sure its the latter) then why risk it. However, Im not trying to shift 50+ departes an hour, sometimes its 5 if I'm lucky!
TIO