This form occurs in Euthyuphro 4b; all dictionaries and grammars, however, give ἐπεξήεισθα, without mention of the possibility of contraction.
A creation of Burnet’s imagination? He doesn’t give an apparatus for this, but Schanz prints ἐπεξῄεισθα, and gives in his apparatus:
ἐπεξῄεισθα B: ἐπεξίησθα T