It doesn't give any extra toolbelts. It gives (up to) 20 extra toolbelt slots.
Just don't research the last tier and you'll end up with 30 slots in all.
There were another toolbelt-mod that put the rows side by side, that way 20 extra slots only used the same space vertically as the vanilla toolbelt. Worked great!
I can't find that mod anymore :\
