This sort of situation is complicated when we deal with single wound models. When dealing with multiple wound models, it becomes even more complex.
Each Base is 1 model, though tomb spyders have 2 wounds and scarabs 3.
Assuming the 1 spyder and 2 scarabs combination, 4 hits should allow the person to force the scarab to be hit through torrent of fire. If this is done at the spyders toughness/armor save or at the majority (scarabs) I don't know. Majority Toughness/Armor save rules would dictate that your rolling on majority toughness of the unit, 3 and not the spyders, but again, torrent of fire is a new rule.
3rd Ed, would have you wounding with 7 bolter hits, removing both scarab bases of 3 wounds each, and taking one save for the spyder, bypassing his improved toughness.
With the Torrent of Fire rule, it could go 2 ways.
1. 4 bolter hits on the unit, he forces you to take one on the spyder, you roll to wound/save seperately from the rest of the shots which will fall against the scarabs.
2. 9 bolter hits on the unit, then taking into account majority toughness, all are wounding T3. The then 6 wounds for example, would kill both scarab bases, but again, Torrent of fire allows the shooter to apply one wound to a specific model, the Spyder, forcing him to take a armor save.
Even without Torrent of fire rules, every model (and multi wound models still only count as a single model) must be allocated a hit before any can be given a second, third, and so forth. Even if the resulting wounds are distributed to as few a # of bases as possible.
... My head hurts...