If the Body does not specifically forbid the stacking of its effects, then you can use multiple bodies at once. Spirit Pulse Dusknoir stacking can only be done in a double battle, as he must be active in between turns. (So the answer to that question of yours is yes.) (This applies to any Pokemon that must be active to use its Body.) Examples of Bodies that cannot stack are Nidoqueen RR, Nidoking RR, Blaziken FB LV.X, Metagross SV, and Shaymin (Land) LV.X (since it says in their card text that multiple effects can't stack).