Why does Beit Shamai require the setting aside of _two_ dishes?
I thought that perhaps two dishes signify a festive shabbos seuda as opposed to the one _one_ dish permitted on erev tisha b'av.
What do you think?
That is a very good suggestion -- the need for two dishes may be because of the festive nature of the Shabbos meal.
The RA'AVAD on the RIF (beginning of ch. 2) writes that the two dishes of Beis Shamai are in order that it be "recognizable" that the food was set aside for Shabbos (the Ra'avad refutes the assertion of the Ba'al ha'Me'or, who writes that the reason for two dishes, according to Beis Shamai, is to have one for Bishul and another one for Tzli). Perhaps the intention of the Ra'avad that it be "recognizable" is as you wrote, that setting aside two dishes makes it clear that it is for the Shabbos meal.