At first I have to make something clear - we DO NOT support multiplication of locos in literal sense of this word. It's just isn't programmed yet. But there's some exceptions and substitutions - for EMU's (EN57 in that case) we have 3 variants of these vehicles as a double version. These are called in the menu "2EN57" (in malature of: Koleje Mazowieckie, PR old paint (dark red-black-white), PR new paint (orange-grey)).
Unfortunately that's all what is officially supported. But you can also (unofficially) make a set with double loco by text editor in set file.
You have to go to C:\Users\USER\Documents\OnRailTeam\TrainDriver2\Presets, find your set and put a second loco by hand. You have to remamber that this way isn't really supported. Even game statute says that dispatcher may refuse to give you timetable if your set does not comply with the rules (2.7.1 of simulator rules), especially when you create a set with 10 locos. You have to know that sets like this can have big issues with breaking, because they aren't supported. With two locos you shouldn't have that kind of troubles, but keep in mind that dispatcher can say no.