How to add extra heating system?

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Zones want to be attached to only a single air loop, but you can have multiple pieces of zone equipment attached to any zone along with an air loop connection - if appropriate.

Thank you for your answer, that is one way to overcome my problem, but I was hoping for some solution that would reduce the 'not comfortable time'.

This is the correct answer in my opinion, but with one little wrinkle. By default HVAC components are autosized in OpenStudio, meaning that first component is going to pick up all of the load and the supplemental space heater will not do anything. To work around this you might hard size the primary system, leaving some unmet hours for your secondary space heater to pick up.

Thank you for the answer. I wasn’t clear enough with my question. My problem is not adding second piece of equipment but how to control working of two pieces of equipment. I have tried @Kyle Benne proposition to hard sized one system (I am not sure how OS chooses system to become primary or secondary system). I end up with nearly the same result but with more unmet hours. I was wondering is there a way to make a schedule in which I could choose when secondary system would start and increase temperature achieved with primary system?