To respond to the last post - if one aircraft is level and another is passing through it's level then I hope that something other than vertical separation is being provided.
I suspect that the original poster is one of the many that have SOPs requiring that the FMS is given the TA on the ay up and the TL on the way down so that among other things there will be a little reminder if the altimeters are not on standard when the TA has been passed and not on QNH when the TL has been passed.
This in general is a good idea as it is yet another posibility of preventing an incident / accident due to for example an altimeter remaining on STD when it should be on QNH.
However, one has to remember that the interpretation of the definition of Transition Level varies from place to place. Many places interpret the situation and provide a TL that is a minimum of 1000ft above the TA. i.e. an aircraft at the TL is always vertically separated from the TA.
Some places have fixed TA and TL.
The UK interprets the TL as being the lowest level above the TA as meaning there is no requirement for the TA and TL to be separated. Therefore it is possible that the TL could be only 1ft above the TA. Yes that was "one".
Therefore in the UK, feeding the TL into the FMS is not going to provide any safety benefit unless the cleared level on the way down is below the TA (in which case you may as well input the TA!!).
In the UK, many places where the holding is at flight levels will have a "minimum stack" level and this level will indeed be a minimum of 1000ft above the TA. Unfortunately while this level is also not on the ATIS, there is a nice table both in the AIP and in the MATS which will enable any crew to calculate what the firtst FL at least 1000ft above the TA is.