Here's
the manual - check pages 6 and 8.
You cannot run an 8 ohm cab with a 4 ohm cab on the speaker outputs, because the total load is in parallel, and those two cabs in parallel are less than 3 ohms, whereas the amp needs 4 ohms - anything below that will fry up the amp.
The bi-amp option lets you use the BT350 as the power amp for either the high or the low frequencies, and you need a separate power amp for the other part - it's all explained on page 8.