Reporting from joint 1099-B to separate returns

I always enter 100% on the person whose SSN is on the 1099, then "Nominee" out the amount reported by the other owner.

Sch B items are shown on the 2nd page of the Sch B entries, Section E for interest and Section D for dividends. Capital gain divs are a little hard to make sure you get right.

1099B is harder, but I enter 100% and enter a second dummy "sale" to remove the profit/loss belonging to the other owner reducing this owner's gains/losses.

This is for the person whose SSN is on the 1099; the other owner just reports half of everything, since the IRS cannot look up their income.

You respond to IRS with the same explanation you gave us:

"The brokerage 1099-B was for a joint account.  However, he and his wife divorced during the year.  So I reported half of the dividends, interest, and capital gains on each 1040 return.  Same with K-1s from a limited partnership.  I clearly marked (50%) on the Schedule B on each tax return.  "

I'd add the exwife's name and SSN in the explanation.

Not that anyone reads IRS instructions, especially when they are directed at the people who don't know they should be filing the form, but here is what the 2022 general instructions for all 1099 forms tell you:

Nominee/middleman returns. Generally, if you receive
a Form 1099 for amounts that actually belong to another
person, you are considered a nominee recipient. You
must file a Form 1099 with the IRS (the same type of Form
1099 you received) for each of the other owners showing
the amounts allocable to each. You must also furnish a
Form 1099 to each of the other owners. File the new Form
1099 with Form 1096 with the IRS Submission Processing
Center for your area. On each new Form 1099, list
yourself as the “payer” and the other owner as the
“recipient.” On Form 1096, list yourself as the “Filer.” A
spouse is not required to file a nominee return to show
amounts owned by the other spouse. The nominee, not
the original payer, is responsible for filing the subsequent
Forms 1099 to show the amount allocable to each owner.

So you're not required to give one to a spouse (does that apply to former spouse, also?  Not clear).  But there's no rule against doing it.  But the time and effort required to issue nominee 1099's is probably more than just attaching a statement, then sending a copy of it when IRS comes back a year later because, you know, who at IRS ever reads attached statements?

The JD Vance solution to this problem is to tell couples to stay married.  Well, at least put your assets in separate accounts for at least a year before going to divorce court.