Wouldn't it be the other way around? The TONE would override the NO TONE.
No Tone has no physical presence to cause any effect, while a tone has some physics behind it and can cause an effect.
A DC Block on the 305 would cause it to receive whatever polarity is selected by the master receiver, since it could NOT send ANY voltage to the LNB.
At least in a power passing splitter, you stand a chance of getting the 18V Horizontal selection voltage through, but like above if the other unit is sending 18V, the 305 sending 13V isn't going to override it.
Since the other receiver is sending a LNB voltage, even in standby, to receive guide and updates, you would at least want to leave it on a VERTICAL 13V channel, so that the 305 could toggle to 18V.
Of course the CORRECT solution is to do it properly with additional lines and proper multiswitches if needed.