is this one of the standard ASHRAE fittings? You may be better off to use a combination of the standard fittings since contractors can make them cheaper and you also have performace date for pressure drop etc.
If you create one out of thin air, you won't have that information, they may be expensive to make (the contractor may make them differently anyway... they have a mind of their own).
You always can create your own family if you really want a non-standard fitting.
I would use one of the double-wye fittings and just manipulate the angles and opening sizes to get something similar.
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