You don't need a separate election. You "elect" it by just reporting the partial disposition.
(B)Manner of making election. Except as provided in paragraph (d)(2)(iii) or (iv) of this section, a taxpayer must make the election specified in paragraph (d)(2)(i) of this section by applying the provisions of this section for the taxable year in which the portion of an asset is disposed of by the taxpayer, by reporting the gain, loss, or other deduction on the taxpayer's timely filed, including extensions, original Federal tax return for that taxable year, and, if the asset is properly included in one of the asset classes 00.11 through 00.4 of Rev. Proc. 87-56, by classifying the replacement portion of such asset under the same asset class as the disposed portion of the asset in the taxable year in which the replacement portion is placed in service by the taxpayer. Except as provided in paragraph (d)(2)(iii) or (iv)(B) of this section or except as otherwise expressly provided by other guidance published in the Internal Revenue Bulletin (see § 601.601(d)(2) of this chapter), the election specified in paragraph (d)(2)(i) of this section may not be made through the filing of an application for change in accounting method.
https://www.law.cornell.edu/cfr/text/26/1.168(i)-8#d
