Hello,
i need help for randomizing 2 questions within the same group of questions.

For example one group of participants get question 1, wherereas another group gets question 2.

I need a step by step instruction please !

Well, under randomizing we understand here that the order of the questions is random within the questionnaire. But you want to randomly assign a question to a respondent, if I am right?

There are many examples here in the forum that describe exactly this behavior. There are plenty of step by step descriptions. If you use the forum search, you'll find plenty.

Here is also a link to a fairly good description of how to do it:
survey-consulting.com/how-to-randomly-sh...a-limesurvey-survey/