can two different material heat treated to the same condition per AMS 2759?

There are general three conditions on these PH grades from suppliers:
cold worked
solution annealed
aged (hardened)

cold worked (heavily) condition is used for CH900.
solution annealed cannot be used for CH900.
both solution annealed and cold worked conditions can be used for H900.

cold work condition can boost mechanical strength (mianly in a strip/wire form)

aged condition is not uncommon. so you donot need to heat treatment if machining is not an issue. also, aged condition helps to keep the shape/dimension tolerance. heat treatment can distort/shrink the final parts

You really want to use these with near zero ductility?
How are these materials going to joined?
Will they be welded together?
If so you cannot use the CH condition for the 17-7 as the heat from the welding will change the local properties.
17-7 is a different class of alloy from the '17-4' types (15-5, 13-8, 17-4).
If it is not being used in the CH condition (heavily cold worked and direct aged) then it requires an austenite conditioning step before aging.
17-7 is most commonly used in the H1050 condition (1400F, cooled and followed by 1050F) or less commonly the RH950 condition (1750F, cooled to -100F and held, then aged at 950F). I have only seen CH900 ever used on springs (coil of flat).
The common solution when these materials are used together is to pre-condition the 17-7, join them, and then age everything at 1050F.
If he doesn't like that strength level then design the whole thing in one material.

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P.E. Metallurgy, consulting work welcomed

Thank you Magben and Ed! It’s a flexible shaft, the end-fittings will be compressed onto the core. How brittle would 17-7PH be at CH900 condition, I guess it is kind similar to springs/coils since the wires are winded with different layers. Both materials are cold drawn. 15-5PH was solution heat treated to H1100 condition prior to the whole assembly aging process. I am still kind lost per my original question...

Tensile elongation of 17-7 in CH900 will be 1-3%.
If this is a mechanical assembly being fabricated from heavily cold drawn 17-7 and 15-5 then the whole thing could be aged at 900F.
If the 15-5 was annealed then you would get the H900 properties.
If it has been annealed and aged at 1050 already, then re-aging at 900 will have nearly no impact on the properties.

Why are you using 15-5? It is a variation of 17-4 designed for thicker sections, usually over 2" thick.
If you need toughness look at 13-8 supertough, it is a remelted grade.

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P.E. Metallurgy, consulting work welcomed

I just realized 17-7PH was solution heat treated and cold reduced to the required size before precipitation heat treat. I am not sure why 15-5ph was picked, but I guess is corrosion resistance and ductility?
So when you see “heat treat the assembly (17-7 & 15-5) to CH900 condition”, you don’t find it odd or anything? To me, it has potentially two meanings, one is to age the assembly using CH900 aging process, second interpretation is heat treat the assembly to CH900 hardness requirement (46 HRC min under 17-7 procedure), which interpretation is the legal one?