Short version: I disagree with your interpretation of UT law, but assuming you have additional facts not in evidence that support it, it's trivial to get the desired presentation.

Long version: I don't have any rental partnerships in UT, but I do have a UT multi-state partnership return, and Utah is kind of weird. I'm working with dummy numbers in the 2019 program, because 2019 works, and multi-state generally doesn't change much from year to year.

I believe that the issue is with the statement that "Income from rental real estate is not considered apportionable income." Do you have a reference to the UT Code, forms instructions, or Tax Commission Rules, to that effect? 

Utah calculates state-source income on Schedule A, and the two pieces that make it up are Line 14 (UT apportioned business income) and Line 9 (Utah net non-business income). If you want something to be UT-source, it has to fall into one of those two categories. Non-business income is allocated; business income is apportioned. 

Tax Commission Rule R865-6F-8 (text at https://silo.tips/download/r865-tax-commission-auditing-r865-6f-franchise-tax-r865-6f-8-allocation-a... ) specifically says "(e) "Business income" means income of any type or class, and from any activity, that meets the relationship described in Subsection (2)(b), the transactional test, or Subsection (2)(c), the functional test. The classification of income by the labels occasionally used, such as manufacturing income, compensation for services, sales income, interest, dividends, rents, royalties, gains, operating income, and nonoperating income is of no aid in determining whether income is business or nonbusiness income."

Also, "(2) Business and Nonbusiness Income. (a) Apportionment and Allocation. Section 59-7-303 requires that every item of income be classified as either business income or nonbusiness income. Income for purposes of classification as business or nonbusiness includes gains and losses. Business income is apportioned among jurisdictions by use of a formula. Nonbusiness income is specifically assigned or allocated to one or more specific jurisdictions pursuant to express rules. An item of income is classified as business income if it falls within the definition of business income. An item of income is nonbusiness income only if it does not meet the definitional requirements for being classified as business income.
(b) Transactional Test. Business income includes income arising from transactions and activity in the regular course of the taxpayer's trade or business.
(i) If the transaction or activity is in the regular course of the taxpayer's trade or business, part of which trade or business is conducted within the state, the resulting income of the transaction or activity is business income for Utah purposes. Income may be business income even though the actual transaction or activity that gives rise to the income does not occur in this state."

Also: "(e) Business and Nonbusiness Income Application of Definitions.
(i) Rents From Real and Tangible Personal Property. Rental income from real and tangible property is business income if the property with respect to which the rental income was received is or was used in the taxpayer's trade or business and therefore is includable in the property factor under Subsection (8)(a)(i). Property that has been converted to nonbusiness use has lost its character as a business asset and is not subject to this subsection."

If you're taking the position that the rental income is non-business income, therefore subject to allocation rather than apportionment, it needs to appear on Schedule H. You enter the income and expenses on Screen 44.132 (Utah Nonbus Inc: Allocated to UT) for the UT piece and Screen 44.133 (Utah Nonbus Inc: Allocated outside UT) for the CO piece. You'll also want to override Rent and Royalty Income [O] for both UT and everywhere in Screen 44.131 UT Apportionment, and any property-based apportionment factors in that same screen.

I agree with your conclusion that Colorado requires allocation, rather than apportionment, for rental income.