Well... either somebody made a mistake, or sources were poor. I'm using T-26 Tank by Maksim Kolomiets (2007). And look at photos and drawing on this page
http://wartechnology.ru/?p=814 - there is only one DT on right side of gun.
I know, where this error might have originated. I didn't want to mess more in previous post, but AT-1 had in fact two machineguns. The other one was carried inside as a reserve, and it could be fired through open side and rear walls. I don't think it is useful, however.
Michal