Unfortunately, olap cubes are pre-aggregated multi-dimensional repositories such that it is not possible to perform simple sql filtering which also includes selections which are themselves a form of filter.
It is possible to have Tableau blend a cube to a relational source such as a flat-file or database table, so long as the cube is the primary object on the relationship, again, this is because Tableau is unable to filter the cube.
With this in mind, if the cube needs to be the primary source on the blend, it will not be possible to place two separate cubes on a blend as both will need to be primary.
This isn't to say that your result is not completely impossible, just that the ease at which Tableau connectes to and filters relational sources is not going to be available.
The tools available to you here will be, using sheets as filters or parameters and calculations working with the Index(), First() and Last() functions.