In a sense, it seems a strange concept to wear hearing aids to boost reception, then wear hearing protection to dull it. There is also the issue of feedback - covering a hearing aid can produce troublesome feedback in my experience. You would then be able to tell who has hearing aids from the whistling sound emanating from them!

If they are in a hearing protection zone, and wearing protection, I do wonder if there is any point in wearing a hearing aid, but then you do want them to hear other things. I came across this problem in call centres, where staff have to wear headphones, and wasn't convinced there was an easy answer to it.

Back at the outset of The Noise at Work Regulations in a noisy manufactuiring environment the best we could offer an employee with dual aids were the ear-muffs normally associated with hearing measurement (adjusted by the manufacturer).

Prompted by your post I have had a quick look to realise little progress has been made other than there are now custom moulded plugs.

Even charities such as action hearing loss seem to be promoting normal commercial solutions.

I think this depends on the type of hearing aid the person uses.  You might be able to get some information from the ENT consultant who supplied the hearing aids or perhaps try one of the charities for deaf people.