whose SSN is on the 1099. if it's yours you may have issues trying to split the income in TT. if you don't report the full amounts per the 1099's (say you are entitled to half so you report only half) you are going to get a notice from the IRS due to 1040/1099 mismatch. on the other hand, if the other person's ssn is on the 1099. you have it easy. just report your share of each item. for schedule D all items will be either type C or F since they weren't reported under your ID number.
The term “partnership” includes a limited partnership, syndicate, group, pool, joint venture, or other unincorporated organization, through or by which any business, financial operation, or venture is carried on, that isn't, within the meaning of regulations under section 7701, a corporation, trust, estate, or sole proprietorship.
I.R.C. § 7701(a)(2) Partnership And Partner — The term “partnership” includes a syndicate, group, pool, joint venture, or other unincorporated organization, through or by means of which any business, financial operation, or venture is carried on, and which is not, within the meaning of this title, a trust or estate or a corporation; and the term “partner” includes a member in such a syndicate, group, pool, joint venture, or organization.
if the IRS deems you a partnership you face substantial penalties for failure to file timely return (with 2 partners about $400/mo from 3/15/2020 the returns due date). you should consult with a tax professional
A joint undertaking merely
