Two Two's

 (עַל־פִּ֣י׀ שְׁנַ֣יִם עֵדִ֗ים א֛וֹ שְׁלֹשָׁ֥ה עֵדִ֖ים יוּמַ֣ת הַמֵּ֑ת (17:6
(19:15) עַל־פִּ֣י׀ שְׁנֵ֣י עֵדִ֗ים א֛וֹ עַל־פִּ֥י שְׁלֹשָֽׁה־עֵדִ֖ים יָק֥וּם דָּבָֽר
Why the change from שנים to שני?  Rav Lazer Silver explains based upon the midrash Massey (23:9) that the word שנים means the two are the same type.  Based upon this this he says the first possuk which is talking about דיני נפשות where the Gemorah Makkot (6b) says that the witnesses must see the act together, from the same view, then the Torah says שנים.  However, the second possuk, dealing with monetary laws, where עדות מיוחדת works, there the Torah says שני for they don't have to see the act together. He says he said this over to the Or Sameach, who approved of the idea.  He uses this idea to explain many other ideas as well, see https://www.hebrewbooks.org/pdfpager.aspx?req=3001&st=&pgnum=231&hilite=.

See also here.