I'm not sure what you mean by 'partners' in a 'self-employed' business.

Partners = K-1

Self-employed = Sch C 

Two brothers formed a partnership for construction work which issues them a K-1. Each pays for their own Medicare, wife's medicare, supplemental insurance. Are they able to deduct these premiums which they could had they filed a Sch C.

You would them on Schedule 1, line 17 of the 1040, "Self Employed Health Insurance Deduction"

Theres a quick zoom button at Line 20z of each partners K-1 worksheet where you can enter health insurance paid.

@Just-Lisa-Now-  Poster said partner paid himself, so would not be on K1

its an entry point to the SEHI worksheets while youre in the K-1 screen, isnt it?

You are right, its an entry point. I don't use the worksheet, unless the partnership pays the premium. Just put on schedule 1, but I also put the foreign tax paid right on schedule 3, as I'm a little lazy 😁