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If I have a gladius strike force of Imperial fists and a skyhammer force of Ultramarines and I choose to enact the Gladius Tactical doctrine, can I enact the Ultramarines devestator doctrine on the same turn? Are combat doctrines one per turn per army, or one per turn per formation?

You may only enact a single Doctrine per turn, not one per detachment.

Where there may be wiggle-room is if you were enacting the same doctrine (i.e. both the Fists and the Um were both using the Tactical Doctrine, thus only one type of doctrine is enacted), but I don't believe that to be the intent of the rule.

There is no wiggle room, a single doctrine is a single doctrine. Tac doctrine from a demi-company is different from a tac doctrine from a um CAD; this is specified with the different sources(if you were looking at the single doctrine as the type, then all tac doctrines would activate on the first declaration; which is counter to the rest of the rules and the example).

You can only activate the demi-company, um, or gladius at any given time, and it will only effect those units.

As another note: the um rule states all ums in the army benefit when "these doctrines" are activated; that means the doctrines provided by the um rule(so a primary detachment demi of if; with an allied um and a um formation woul have 2 sets of doctrines, 1 from demi and 1 from ct:um, but the ums would only benefit from the um set, not the demi set)

For an exact quote, see the last sentence of the opening paragraph on Combat Doctrines:

"You can enact a single available Combat Doctrine at the start of each of your turns."

Kommissar Kel is quite right that in the case of the Gladius its available Combat Doctrines affect only the units in the Gladius, and those provided by the Ultramarines Chapter Tactics affect only Ultramarines units.