If you’re new to Dynamo:
- Take the time to read the Dynamo Primer: https://primer.dynamobim.org/
- Read the forum guidelines on asking for help: How to get help on the Dynamo forums
Programming is a not a forgiving paradigm if you do not show evidence of at least trying to learn and attempt solutions before asking others to give some to you. Your motivator into wanting to code should be to eventually help others rather than simply receive help. My reply is overly vague as you have not indicated an attempt at the problem, and I suspected you are likely very new to Dynamo and skipped the training wheels we always recommend to newcomers.
As a guide to how you could try to solve the above problem, find a way to sort the mullions. You’ll need to filter them down to those that belong to that system, then all which are parallel to a selected one (too hard for Dynamo to read your mind! It needs some help in knowing what to work with). From there sort them in a linear order based on some geometric properties they hold (e.g. XY location), and then split them into lists of 1 and 2 (partition list can do this), then unpin/delete the sets of 2.