Now that's extremely weird, although we only have a few details of what actually happened.
Besides being explicitly forbidden by the sacred texts (ICAO Doc 4444, para 12.2.7 "The aircraft or vehicle causing the condition in the clearance issued shall be the first aircraft/vehicle to pass in front of the other aircraft concerned"), giving a conditional line-up clearance behind the second landing traffic would be nothing but looking for trouble. Also, the need to specify both a/c types and positions seems to defeat the purpose of a conditional clearance, ie. making things more efficient and optimise RT use. Not to mention that your mind would have to remain focused on the holding point longer than normal, possibly overlooking other items in your priority list.
I once issued a conditional line-up clearance to a Falcon 2000, at night, CAVOK, with a sequence of 3 aircraft on the localiser, well visible from the holding point. I had a pretty good picture of the situation, but the crew didn't (which is absolutely normal: they don't have a radar, and it's not easy to evaluate distances at night), and they showed good airmanship by challenging my clearance and asking "Confirm it's a lineup behind the first landing traffic?". Even though a line-up behind the second aircraft would've been forbidden by the rules, they had a very valid point, IMHO.