Is the prayer invalid in a mosque where there are two walls separating the prayer area of the worshipers from a grave that is in the direction of the Qibla, knowing that the grave is not a shrine and is not sought for blessings?

There is no objection to praying in this mosque, because the two walls between the mosque and the grave are a barrier that prevents the worshiper from praying towards the grave. If a single wall is a barrier, then two walls are even more so.