You could create a duplicate from the other layer and place it exactly in place on the top most layer. Then create the trap (everything spreads into black, yellow always spreads, mag and cyan split), then keep the trapping elements and either move them back to the first layer or turn off the original and save as Trp. There is always a manual workaround; in 33 years, I have yet to be stumped but I can tell you, there have been a few doozies.
Of course you need to trap in Photoshop as auto traping systems will not trap within an image and certainly trap is required - eg: pure yellow photoshop type or object across an underwater scene that is void of yellow.
John W