At the coming (epi tēi parousiāi). At the coming here of Stephanas, etc., the very word used of the parousia of Christ (1Co 15:23).
That which was lacking on your part they supplied (to humeteron husterēma houtoi aneplērōsan). Either “these filled up my lack of you” or “these filled up your lack of me.” Either makes perfectly good sense and both were true. Which Paul meant we cannot tell.