Usage: 2 forms in 2 verses
Form Translated As     In Verse(s)
συνεμάχησαν fought along with LXX.1Chr.12.21
συμμαχήσετε you shall fight along with LXX.Josh.1.14

Liddell-Scott-Jones

συμμαχέω (summacheō) to fight along with G:V

συμμαχέω-έω,

aorist συνεμάχησα [Refs 5th c.BC+]: perfect συμμεμάχηκα [Refs 2nd c.BC+]:—to be an ally, to be in alliance, [Refs 5th c.BC+]; οὐ ξ., ἀλλὰ ξυναδικεῖν join not in war but in doing wrong, [Refs 5th c.BC+]: generally, help, succour, σ. τινί [Refs 5th c.BC+]; σ. ὥστε.. assist towards.., [Refs 5th c.BC+] — middle, perfect participle συμμεμαχημένος in active sense, [Refs 2nd c.AD+]:—passive, συμμαχοῦμαι ὑπό τινος [Refs]