First of all, within the actual game you can only connect vessels for towing jobs when you are in the mission editor mode. In game its not possible until now, but it will be a new feature for SS08.
The editor asks you, which vessel should be the towing one and which should be the towed. So its a little bit funny, if you want to bring two tugs into the game, one at the front and the other aft, you have to choose the following: bow tug tows vessel (for example the container freighter CV) with one rope; than you have to choose CV tows tug at the stern. Weird, uuhhhhhhh? Last but not least, the stern tug will be attached with the rope at his stern, so its better to turn him backwards right within the editor mode. The Russian tug from the Add-On could tow with its bow winch as well, but this is not very realistic with this type of tug at all.
Cheers