Wa alaykum salam wa rahmatuLlahi wa barakatuHu,
It is not permissible to house two wives in the same space, even for a single night, unless they are happy with that. If they are happy, it is permissible; Ibn Abi 'Asrun qualified the As-hab al-Awjuh's itlaq with this detail.
The reason for this ruling, as Imam Rafi'i mentioned, is to avoid discord. As if they are made to stay in close quarters, it may well lead to bickering and other problems.
In many books of fiqh, this obligation is termed 'iskan,' which literally means 'providing living space.' And much of the basis for determining what constitutes having fulfilled it is custom (ar: 'urf). Between providing 'living space' and providing a 'house' are certain similarities and also certain differences. Saying a 'house' is due may exclude other things that would suffice; especially if we consider what may be customarily intended by the word 'house' nowadays.
In Rafi'i 8/364 and Rawdah 7/348, Shaykhayn mentioned the case of one having a house with multiple living spaces inside and stated that it would be permissible for him to house his wives there. They mentioned a two-story housing situation, one wife has the top floor and the other the bottom. The issue of the house's structure is not their point, rather the presence of separate spaces.
Additionally, amenities must be separate. If this is the situation, i.e. they have been provided a living space with separate amenities, then housing them together in one house is allowed and they have been given their right of 'iskan.' This detail has been conveyed in the Madhhab's reference works throughout the eras, in various chapters, so its foundation is well established in the Shafi'i School.
The term the As-hab al-Awjuh use for amenities is 'marafiq.' They mention examples of amenities as things like a place to relax, a kitchen, access to the roof, etc. Some of these are stated by Shaykh al-Islam in Sharh al-Rawd 3/231.
Some things mentioned may have been necessary considerations in earlier times or in different places. A woman needing access to her roof in suburban America, for example, is a bit out of place. While on the other hand, a toilet and restroom facilities (i.e. modern indoor plumbing) would be essential.
The examples, even if some perhaps seem archaic, provide a good indication of what the As-hab intend by: separate living spaces with amenities included; or as Ibn al-Muqri and Shaykh al-Islam stated, 'bi maskan la'iq biha wa law bi hujurat tamayyazat marafiquhunna.'
Again, 'urf plays a role in determining the living space that is suitable for a married woman. And 'urf is ghayr mundabit [not easy to pinpoint] and subject to change depending on different peoples, places, and times.
Allah knows best.
Answered by Shaykh Yaqub Abdurrahman