What capacity must the heat pump have?

The system is 4.5m away from the neighbor's house, so acoustics are not entirely unimportant. Is it sensible to use a heat pump like we currently have in our two-family house (two stories), or would it be better to have two smaller ones that each supply one condominium?

If these are two separate units, two heat pumps and two separate hydraulics are advisable. You can then operate two heating curves without a buffer tank or ERR, and the residents can set the apartments to different temperatures. Especially if you rent them out later, this otherwise causes problems. If you buy them yourself, the heat pumps only cost 2x €3,000.

Regarding noise:
Only at 60% heating output (approx. 3kW) and simultaneous subzero temperatures does it become audible. Above that, the noise level increases significantly; below that, it is practically inaudible. Nevertheless, I would install a privacy screen toward the neighbor directly or even earlier, because what you see you hear, and visually the Panasonic is not a highlight either. During drying, it ran with 5kW for a long time at our place, and it stands acoustically unfavorable in a corner here; the neighbor could hear it at night (20m, old windows).

Ours is now limited by whisper mode to about 60% output (approx. 3.3kW); with defrost cycles it does not manage more than 2.5kW on average during the day. I let it run freer (4kW) for a few hours during the day by timer. You cannot limit it below 3kW, meaning that it is slightly audible in any case when it’s cold, but the difference compared to 5kW output is very large. In general, Panasonic is considered very quiet. I don't know other heat pumps and can only report from ours.

The idea of two smaller heat pumps for each household is off the table since the smaller devices start at 5kW and that would not be suitable in our case. That was just a thought.

On the contrary, the idea is excellent. Do it that way! Oversizing is a problem with non-modulating or poorly modulating heat pumps because it leads to many heating cycles. The current Panasonic 5kW model (H in the type designation) modulates down to about 1.3kW heating output (at <200W power consumption). If you lower the heating curve at night by timer, which is recommended anyway, it won't start at night during transitional periods at all. This effectively halves the heat pump’s capacity and places the possible noise load into the day. It will not cycle if the rest of the hydraulics are correct. My observation is that it runs not only quieter but also more efficiently in the modulation range.

Alternatively, you can design the hydraulics so that you first try with one heat pump and later completely separate the two hydraulics with a shut-off valve. For that, however, it's better to visit the HTD forum.