Is it possible to locate the 'main' router very close to, or perhaps just outside, the window?
Is the 'main deck' a potential location for the single router?

If there is a stairwell that leads to the lower unit, or is there a location that would present a radio path with less obstruction?

The thinking is that a single WiFi source may better 'illuminate' both residences if the location of the main unit is rethought.

In general I have found that within a 'typical' residence the WiFi/router unit when located at head height or a little higher provides generally good dispersion.

In your situation I suspect that most of the radio propagation is occurring through windows, doorways, and other openings in the structure. Especially between floors. So think about where there are 'thin spots' in the structure, from a radio perspective.

The radio signal will of course also be affected by large metal items which can create 'radio shadows'. Metal ductwork, large appliances, metal desk, metal doors.

Without a rather thorough 3D model of the structure it can be difficult to estimate how successful a single router might be, even one with amped up radio specs.