Thanks a lot for your rapid and useful answer. I actually thought about this solution of creating two exercises.
But then I was wondering how sawtooth ascribes the versions (block) to each respondent, that is: could it be that one respondent faces version 1 in the first exercise and version 2 in the second exercise?
If this is the case, then I have a problem because I want 2 alternatives to appear identically in the 2 choice tasks. (This is also a problem if the tasks are not always displayed in the designed order, that is, exercise1 shows first task 1 and exercise2 shows first task 3 for instance).
If this is not the case (and it seems to be not the case when I use the preview), then my problem is solved, thanks a lot!