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I want to make an Astra Militarum army but I"m confused about the detachment and doctrine rules. I want to have a main detachment that is Militarium Tempestus Scions and a Catachan Detachment. If both detachments are only using their own units can they be played as part of the same army and still get their regimental doctrine?

Same army, yes. Same detachment, no.

Have you read the rules in the codex on building detachments?
P132.
Militarum tempestus units don't stop other units gaining thier regimental doctrine. But the entire detachment has to be militarum tempestus to get thier doctrine.

They can be played together. If played on this way, you will only recieve the Catachan doctrine on your Catachan units. Militarum Tempestus do not block other doctrines, but will not benefit from their own.

Edit - I missed that there were 2 detachments. I think each should get their perks as long as the MT one is entirely composed of MT, ie no Catachans hanging around.