I'm still gonna go with my original theory, I very much doubr there playing, its more likely the dominant one is getting annoyed by the other shark and is trying to chase it away. Almost any website you care to read on Rainbow sharks clearly says keep them alone (i.e. one shark) or keep a group of at least 6, do some researxch yourself, as I say I've recently got one too and tried to find info on these fish, to be honnest its not very much, for some reason theres a general lack of info especally on rainbow sharks. However from the scraps I have read, you need substancial tank room because the thing about all these sharks is thee are teretorial and like to establish certain areas of your tank as their own, say a piece of pipe or driftwood etc. I've no idea what size these sharks are of yours I presume they are still young ????.
Like I said in my last post they do generally get more agressive as they get older, I notice you say you have two tanks, why not put one of your sharks in the new tank. Even if not for the sake of them fighting each other their eventual size will mean they outgrow 14/16g tanks quite quickly and really you'd be looking at a 40-50g tank just for each fish. But anyws have a look at this link it seems to be the most helpful of anything I can find.
http://www.****************/catalog_pages/sharks_eels_loaches/shark_rainbow.htm