Thank you both for your answers.
I understand that services can be provided remotely, and I agree that the name of the controlling unit can be completely unrelated to its location or the location of the airspace controlled (Frisbee tower controls Geilenkirchen airport).
Would the delegation of airspace be included only in the units' LoAs with each other? Should it not also be included in the AIP?
Let's say that a VFR flight wants to cross the CTR/TMA at 2.000ft from the west (Belgian class G airspace) to the east. Should the pilot contact EHBK TWR or EHBK APP?
To eliminate the difference in airspace classification, we could look at LOWS. There, both CTR and TMA are class D (between 3.500ft and 7.000ft) and the CTR is again within the TMA.
I had a look at Annex 11, and the only related thing which I found was this:
2.9.5.3 If a control zone is located within the lateral limits of a control area, it shall extend upwards from the surface of the earth to at least the lower limit of the control area.
Note.— An upper limit higher than the lower limit of the overlying control area may be established when desired.
So while it says that such a structure is possible, it does not resolve the ambiguity of the responsibility in the common airspace.