Does anybody know if the one handed perks effects 'savage strike' (+25% damage on power attacks) and dual savagery (+50% damage on power attacks with dual blades) are cumulative?

I don't think they should but I've started seeing decapitation finishing moves with two weapons, something I didn't notice before taking this perk (which, in addition to granting a 25% damage bonus it also causes random decapitation)

I'll do a quick test now. Editing with results in a bit, if my computer doesn't do a Normal Shutdown.

Test results in. Method:
Target: Heimskr, setunconscious 1, forceav health 2000.
Weapons: 2x unimproved Steel Maces (base damage 10; skill 100 with 5/5 Armsman, 30 total damage; no enchants or potions.)
Perks: Full 1h tree, excluding Mace spec. (Character has been testing other aspects of melee.)

Normal attack: 30
"Dual wield" normal attack: 60 (expected for 2 x 30)
Side and back power attacks, using one hand: 60. Neither perk is applying.
Forward power attack, using one (either) hand: 90. Dual Savagery is applying, but Savage Strike (predictably) isn't.
Critical Charge: 102.5. This is 90 + 12.5 (which is actually expected for 2x critical damage at 10 base.) Dual Savagery applies again.
Standing power attack, using one hand: 112.5. 60 x 1.25 = 75 x 1.5 = 112.5. Both are applying.

For the Dual Wield Power Attack, I removed Dual Savagery and Savage Strike:
No perks: *135* damage. Each of the three hits was only gaining a 50% power attack bonus, as this is 45x3.
Savage Strike only: 168.75. This is the expected 25% bonus.
Both perks: 253.12. This is actually 253.125, 50% more than 168.75. Both are applying.

The biggest oddity is why forward and sprinting power attacks get Dual Savagery, but side and back power attacks don't.

Wow! Thanks for the in-depth anolysis.

So, to sum up:

One handed, standing: both apply = Power attack (2x normal damage) * 1.5 * 1.25 = 3.75x base weapon damage
Two handed, standing: both apply = Power attack sequence ( 4.5x damage of one hand, assuming both weapons equal) * 1.5 * 1.25 = 8.4375x base damage

Base weapon damage has a specific meaning: I'd go with "3.75x normal attack damage" instead.

As for the dual wield power attack, it's two left hand hits and one right hand hit. So if the weapons are unequal, the anolysis is a bit different:
L = Normal hit damage left hand; R = Normal hit damage right hand.
((2*L+R)/3) = A, average damage of one hit;
A * 8.4375 = actual damage.

In other words, (2*L+R) * 2.8125.

Example: With a glass mace (14 base damage, 42 NH) left hand, and a steel mace right hand:
Each glass hit is doing 42 * 2.8125 = 118.125
Each steel hit is doing 84.375, as noted above (30 * 2.8125)
Total: 2 glass and one steel = 236.25 + 84.375 = 320.625. (For some reason, I'm getting 320.875 as the actual damage in this test. Close enough.)

But if the glass mace is right hand:
168.75 + 118.125 = 286.875.