If you want to split your process into two branches STEP1 and STEP2, you need a process variable to be tested to choose the way to take.
You want to add a variables like 'choice' type integer.
You want to split your process with an XOR gate adding conditions on the branches that leads to STEP1 and STEP2. choice==1 on the branch that leads to STEP1 and choice==2 on the branch to STEP2
In your form, you will add either a dropdown list to choose the value of the variable 'choice' or you can add two SUBMIT buttons, with an action that sets the value of choice to 1 or 2.
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