Easiest Way is to make Two DTs. Its very ovious that you have two picks that way.
More than likely your looking for a differant answer though so here...
In your triggers that create the units your picking make sure you move the DT back to a respawn place. In addition add one to a counter (deaths, minerals, custom whatever). In the conditions of the same trigger require that there be 1 or less for the counter (at most). Using this method you could can give the player any amount of picks you want.
Alternatlively if you reverse the counter so it counts down you could add another step that lets the players pick how many choices they wantExample:
| Trigger |
| Players: |
| ¤ Human Player |
| Conditions: |
¤ Current Player brings at least one DT to 'unitA'
|
| ¤ Current player suffers at most 1 deaths of 'unused unit' |
| Actions: |
¤ Create one unitA for current player at 'feild area'
|
¤ Move All DT at 'unitA' to 'respawn'
|
¤ Set Deaths of 'unused unit' to add 1 for current player
|
| ¤ Preserve Trigger |
All your other spawn triggers would be the same making sure you use the same unit for your counter.