The (Arab) Joint List hints at the possibility of supporting a centre-left–ultra-Orthodox coalition. Such cooperation sounds quite exotic, and I understand that the offer comes from the Joint List's determination to oust Netanyahu.
But is there any indication for such a scenario at all? What are the red lines for Shas and United Torah Judaism on the one hand, and for the Joint List on the other?
While such a scenario was proposed by the Joint List, it is not currently considered by any of the other parties included in it.
In general, the ultra-orthodox and the Arabs are concerned with completely different interests, so their red-lines don't conflict at all. The orthodox are interested mostly with the continuous funding of their voters (Yeshivas, high child allowances) and religious aspects of the public life, such as public works and trade on Saturday. The Arabs are interested in promoting the economic state of the Arab population, fighting crime in Arab towns and villages, and moving the political process with the Palestinians forward. So in theory, they can support any left-wing government that includes the orthodox parties.
Edit in reply to comment:
Yahadut Hatorah has no interest in the West Bank, except maybe the few Haredi settlements, most of which are just across the Green Line (1967 armistice border). Shas is more right-wing inclined, but will likely agree to a withdrawal if they are convinced it is a real and viable agreement.
Such a cooperation isn't considered because the orthodox know they'll get what they want/need from Netanyahu, while with Kahol Lavan they'll have to give up much of their achievements. So they rather stick with him, as long as Kahol Lavan can't create a coalition. When time comes that they need to choose between the opposition and joining Kahol Lavan, things may change.
As for Shas, I think it is important to recall that they showed a lot of support for Netanyahu in the last election campaign.
For example, they actually featured pictures of current party leader Deri alongside Likud leader and current PM Netanyahu - this is an extreme show of support, as far as national politics are concerned (putting both yourself and Netanyahu on the banner is something you'd expect a Likud-affiliated mayor candidate to do).
So, it can be viably said that Shas has a red line - a coalition led by Netanyahu, not giving in to a coalition led by Kahol Lavan (especially with the secular Lapid as one of the party leaders).
