ilmiont

Ye, as I wrote above, I've ended up using transparency on a duplicate layer to do what I want.
Imagine you have a 2D bird in flight in two unions, body/wings and neck/head.
I need a black radial gradient starting at the body and ending in grey at the wing tips, but white at the head.
To appear realistic, both the grey/white areas have to appear to be on the same path from the black area of the body. So really, I need a hypothetical V-shaped gradient path, with black, white and grey stops, where black is shared.
I've achieved the effect I want by applying the black-grey gradient to the body and the black-white gradient to the head (black slightly adjusted on the head to be closer match to the colour of the gradient body pixels near the join between the shapes.)
Then I duplicated the layer, joined both shapes and applied the black-grey gradient across the entire thing. Then a careful transparency curve allows the white on the lower layer to show through, while preserving the smooth gradient on the neck.
On mobile now, but tomorrow I'll add examples to illustrate. Struggling to describe the effect! Important point is, there are two objects, which need to have a gradient that makes them look like one object, but their end stops have different colours.