Ventilation system for a two-family house

Hello, we are currently building a house with a main apartment (160 sqm) and a separate apartment (70 sqm).
A ventilation system with heat recovery is planned. The architect wants to design two separate systems, the heating contractor says that one large system is also feasible.
My questions:

[*]What are the advantages of two separate systems
[*]How is one large system billed between the residential units? I would calculate the ongoing costs based on the ratio of the living areas. But can the heat recovery be taken into account? Do the costs even play a role?
[*]What happens if one residential unit goes on vacation and does not heat? Will the other apartment then be unnecessarily cooled by the cold extracted air?

Thanks in advance for the answers

Regards, Arthur

1. No sound transmission between each other, full cost control, very simple regulation Question 2 and 3 are unnecessary.

2. Difficult to regulate, unclear costs, heat recovery negligible.

3. No problem at all... because the air exchange is not high enough for it to matter... or the other apartment must still be kept at a certain temperature... dew point!

Hello,

..... The architect wants to plan two separate systems, the heating engineer says that one large system is also feasible....

The architect is correct in terms of defect-free construction contract law. DIN 1946-6 refers to usage units. Two apartments, two controlled residential ventilation systems, which must function independently of each other.
In theory, this could also be achieved with one large system, but only with disproportionately high control engineering effort. The system becomes significantly more expensive. Whether the heating engineer has taken into account the required functional independence in his "large" system, I rather doubt.
The room-specific air volumes have, especially in well to very well insulated buildings, sometimes a considerable influence on the performance balances. It is not without reason that in the heating load calculation the room-specific air volumes according to the air volume plan according to DIN 1946-6 must be considered.
Therefore, be sure to think of the measuring protocol after installation!
An alternative variant would be: main apartment => central controlled residential ventilation, secondary apartment => decentralized individual solutions.

Best regards