If you got an 8 ohm cab a simple rule to have would be "1 in 1 out", in that order. That way there is always at least one cab connected and impedance is always in the range. No problem being interrupted mid process.
Actually, not sure that works as the second jack is labelled extension with no switch. There could be internal switching on the jack itself so equally possible there is already self shorting on the main one, which would make it all a non issue. Schematic time.