The headword, illustrated by both of the quotations given, is the present active participle, neuter nominative/accusative singular, of the verb sune/xw, I hold/keep together, unify, succeed; see generally LSJ s.v. and cognates at sigma 1523, sigma 1524, sigma 1525, and sigma 1526.
[1] Polybius fr. 226 Büttner-Wobst. The fragment is unplaced but very similar to Polybius 33.4 (web address 1), already at alpha 3919. (That passage, however, does not contain the present passage's opening words to\ de\ sune/xon, which should be understood adverbially.) If this association is correct, the fragment describes the Rhodian navarch Aristokrates and so pertains to events of the war between Rhodes and Crete (late 155 BCE or 154); Walbank, p. 545-6. [In her critical apparatus Adler reports that ms G lacks the conjunction de\: [they had] a unifying leader...; also that ms F replaces h(gemo/na kai\ with h(gemo/nas, but by unifying leaders, a completely capable commander.]
[2] Polybius fr. 227 Büttner-Wobst. The fragment is unplaced and is omitted by ms F (so Adler). Also, Adler here (apparatus) follows Dindorf in transmitting the optative ei)sde/cainto (they could accept). However, with Büttner-Wobst (p. 544 note) she observes that grammar requires the particle a)\n for a potential optative; cf. Smyth §1824. In fact, in the absence of this particle Büttner-Wobst (ibid.) suggests that the Suda might well have given ei)sdeca/menoi: neither soldier nor junior officer were at all accepting (aorist middle participle, masculine nominative plural, from the verb ei)sde/xomai, I admit, take into; see generally LSJ s.v.)