Where I disagree with Gryllus is to see μεθ’ ὑμῶν as adverbial on its own. If it is to be taken on its own, it must stand in place of a predicate nominative as the main clause, because εἰς τὸν αἰῶνα will then become a dependent adverbial clause, which acts on that main clause.
For I disagree with Gryllus that "εἰς τὸν αἰῶνα" can be applied directly to the verb alone. Although adverbial ( a time clause), it is a dependent adverbial clause, that qualifies the whole of ᾖ μεθ’ ὑμῶν. μεθ’ ὑμῶν with ᾖ is acting as the main clause. The substance of the communication is that "I may be with you."
As I said, I think the better way is may be to see the whole of «μεθ’ ὑμῶν εἰς τὸν αἰῶνα» is as standing in place of a predicate nominative rather than adverbial, but it makes little difference if you take whole clause as one clause. It's where to you hive off εἰς τὸν αἰῶνα as an independent clause that things become complicated. However I agree the idea of two independent adverb clauses following each other in succession makes little sense.