I'd feel safe in assuming that many people do that without issue, as long as the contact integrity of the main speaker connections is not compromised. (How the sub wires are physically connected is way less critical, since the sub draws almost no current from the power amp).
As a point of info, though, I would see no problem with connecting the + wire from the sub to the 8 ohm tap while connecting the + wire from the main speaker to the 4 ohm tap, although of course that would still leave you with two wires connected to the ground terminal. Doing that would result in the sub receiving a slightly higher input voltage (about 3 db higher), which would be compensated for with its level control.
Regards,
-- Al
As a point of info, though, I would see no problem with connecting the + wire from the sub to the 8 ohm tap while connecting the + wire from the main speaker to the 4 ohm tap, although of course that would still leave you with two wires connected to the ground terminal. Doing that would result in the sub receiving a slightly higher input voltage (about 3 db higher), which would be compensated for with its level control.
Regards,
-- Al