That sense was added in 2007 by Paul Blay with the reference: "第１条 甲乙双方は、信義を重んじ、誠実に本契約を履行しなければならない。" and the comment "Generally in legal contexts. Hard to tell how common it is but 5,560 hits for 甲乙双方, 18,600 for 甲乙間."
Googling for 甲乙双方 brings up a lot of legal stuff. I suspect it's probably overdoing it to say it's a legal term per se. I think sense 1 is probably enough. 甲乙 is not in my legal glossaries.

@@ -28,5 +27,0 @@
-<sense>
-<pos>&n;</pos>
-<field>&law;</field>
-<gloss>first and second parties</gloss>
-</sense>

prog, daij, gg5

is sense 3 correct? not in any of my refs. prog has "the former and the latter" though.

@@ -17,0 +18 @@
+<gloss>first and second</gloss>
@@ -19,2 +19,0 @@
-<gloss>excellent and good</gloss>
-<gloss>similarity</gloss>
@@ -23,0 +23,3 @@
+<gloss>superiority and (or) inferiority</gloss>
+<gloss>distinction (in quality)</gloss>
+<gloss>discernment</gloss>
@@ -25,2 +26,0 @@
-<gloss>discernment</gloss>
-<gloss>distinction</gloss>
@@ -30 +30,2 @@
-<gloss>first and second parties (in legal contexts)</gloss>
+<field>&law;</field>
+<gloss>first and second parties</gloss>