Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
There is no harm for a married couple to have a bath together in the same basin.
Bukhari and Muslim reported that each of Ayisha and Um Salama (Radiya Allahu Anhuma) narrated that “I used to have a bath with the Prophet (Sallallahu Alaihi wa Sallam) after Janaba (ritual impurity) in the same basin”.
This is about having a bath after sexual impurity in general. As for what is forbidden for a married couple during fasting, it is to have sexual intercourse or indulge in foreplay which could cause ejaculation of sperm or pre-seminal sperm.
Therefore, if one does not see in the other partner’s body what could lead to that, then there is no harm for them to have a bath together, otherwise no.