That's because in the teaser trailer for the season finale, which aired last night following "Flash vs. Arrow," it seems that both men are present in the room when the Reverse-Flash goes after S.T.A.R. Labs.
Update: I'm told the trailer did not air during The Flash in most markets, although it obviously did somewhere, since low-quality images from this teaser appeared online immediately following the episode last night.
What else do we learn about him? Well, he's in quite a bit of the episode, from the look of things; this shouldn't just be a simple cameo to whet your appetite in the last thirty seconds, a la last night's appearance of Firestorm.
Over at Flash TV News, they've got a pretty extensive gallery of high-quality screen captures, from which these two images were taken.
You can check them out for different angles and to hunt for clues as to which Reverse-Flash it might be or who could be under the mask.
The Flash airs Tuesday nights at 8 p.m. ET/PT on The CW, but Barry Allen will appear on Arrow tonight at 8 p.m. ET/PT on THe CW.