Flat roof joists resting across both inner and out skins? (total newbie)

I'm building a rear extension with the joists running across the back of the house (not out from). It's going to be a cold roof with ventilated soffits at each side. So far I've built both skins to be exactly the same height, and I was planning to put wall plates on both and run the joist across both.
I (now) know that the 'usual', 'traditional', 'correct' way is for only the inner skin to bear the weight of the roof.
My question is why, what's the reason? What would the negative consequences or dangers be of using both skins?
I just want it to be as strong as possible. They are 9b3 and almost 6m long over 5m span, my inner skin is Thermalite, the outer skin is 7n concrete and I feel it would be a shame not to use their strength.
But I'll do whatever is correct.
Any advice will be gladly received!
Cheers

Normally the joists are what the fascia and any soffit is fixed to and support the roof deck right to the roof edge, so do cross both leafs.

^woody^ said:

Normally the joists are what the fascia and any soffit is fixed to and support the roof deck right to the roof edge, so do cross both leafs.

Click to expand...

Cheers Woody,
That's what I want to do but I can't find a single example of anyone doing it anywhere on the internet.
I have no experience of building or regs...I'm just researching each step of the build as I go and doing everything myself (because I don't have the money).
I'm just worried there's a structural reason why people don't do it.....and I don't want to find out the hard way!

I think it is probably usual just to have wall plate on internal leaf because that is what you would have to do for a pitched roof.

There is no wall plate on a flat roof.

Don't you have any drawings to build to? There must surely be some details on the Internet if you search.

The joists are the roof structure, so they extend to the edges of the roof. Where the joists are parallel to the one wall, fit noggins every 600mm and you have the first joist on the outer side of the external leaf.

Twisted straps at max 1500mm centres to hold the joist ends down.