Firstly I havent a clue how an 'Openbox' works but on a Humax, you open the Menu to set all the satelitte names and frequencies and searches as well as setting the DsQ thingy.
Now DsQ thingys I think are 'labeled' A-B-C-D, in the Menu to set up say C1 and turn ON and Enable the DsQ thingy and set as 'A', you then repeat the whole thing and set 'B' for D1 or D3 whatever.
Its entirely your choice what setting you want for each dish.
DsQ thingys come with 2 or 4 connections from memory not 3 so just ignore D and cover it so water doesnt enter.
Now what should happen is when you change channels, the STB sends the appropriate signal to the DsQ thingy for it to select the correct Feed, BUT dont be surprised if its a bit slow switching.
As you said you have both dishes set up and receiving, now you have to Enable the DsQ settings in the Menu.
I cant remember now but I thought 'A' is the default setting and would 'Pass thru' any signal even when not enabled in the Menu.