To briefly summarize how maps in ETS2 work, each map is built on a .mbd file ("Map Binary Data" file iirc). The Defualt Europe map is built on europe.mbd, and any map mod that connects to/edits the default Europe map is also built on the europe.mbd file. Basically, if two maps both share the same .mbd file, they can both be loaded at the same time. This is how 'map combos' for the default map work, they're just a collection of maps all based on europe.mbd.
So with that knowledge, the same technique can be applied to standalone maps, as long as the both share the same .mbd file name then they can both be loaded at the same time on the same profile.
So to make FGS and Pomezania work with Grand Utopia I've simply changed the .mbd file of FSG and Pomezania to be the same as Grand Utopia's .mbd file. (grandutopia.mbd) Add in a few ferry connections and the result is the ability to play Grand Utopia, FSG and Pomezania at the same time, on the same profile. simple!

Credits: bricksathome