Greek: ἐπιποθῶ γὰρ ἰδεῖν ὑμᾶς, ἵνα τι μεταδῶ χάρισμα ὑμῖν πνευματικὸν εἰς τὸ στηριχθῆναι ὑμᾶς, τοῦτο δέ ἐστιν συνπαρακληθῆναι ἐν ὑμῖν διὰ τῆς ἐν ἀλλήλοις πίστεως ὑμῶν τε καὶ ἐμοῦ. (Romans 1:11-12 WH)
NWT 2013: "For I am longing to see you, that I may impart some spiritual gift to you for you to be made firm; or, rather, that we may have an interchange of encouragement by one another's faith, both yours and mine."
ESV: "For I long to see you, that I may impart to you some spiritual gift to strengthen you—that is, that we may be mutually encouraged by each other's faith, both yours and mine."
For Romans 1:12, the Weymouth New Testament states: "in other words that while I am among you we may be mutually encouraged by one another's faith, yours and mine."
The Expositor's Greek Testament calls τοῦτο δέ ἐστιν, "an explanatory correction." The Cambridge Bible observes that Paul is using tact at 1:11-12 in order to combine sympathy with judgment: he wants to clarify that he will not only encourage the holy ones in Rome (1:7), but they will strengthen him too.
From Richard Longenecker's Romans commentary: The second statement of 1:11-12 begins with the expression τοῦτο ἔστιν (“that is”) and the postpositive connective δέ (a mildly adversative “but,” though here probably best translated simply “and”), which together signal an explication. So this second statement is meant to clarify and expand on the immediately preceding statement.
Robertson's Word Pictures: That is (τουτο δε εστιν). "An explanatory correction" (Denney). The δε should not be ignored. Instead of saying that he had a spiritual gift for them, he wishes to add that they also have one for him.