No. A Unit consisting of a Single Model may never assault more than one unit.
This is because of the way the assault rules work, in that within the charging unit, the closest model to the enemy that is the declared target must move into
BTB contact with THAT unit only. It is only then that the rest of the unit get a chance to engage other units. Single Models are still units, albeit units of 1 model, so do not have a chance to multi assault.
In the situation you describe there, you would have to declare either the wave serpent or the Dire Avengers as the target for your assault, then move into
BTB contact with the closest model/ point while also remaining more than 1" away from the other unit, so it is a moot point anyway since it cannot be legally in
BTB contact with both the transport and the unit at once.
And dont forget that you have to be in
BTB or within 2" of a model in the same unit to get any attacks.
And now a couple of quotes to back up my babbling:
Page 34 wrote:All of the models in an assaulting unit make their assault move following the same rules as in the Movement phase, with the exception that they may be moved within 1" of enemy models. This means that assaulting models may still not move through friendly or enemy models, may not pass through gaps narrower than their base, and may not move into base contact with enemy models from a unit they are not assaulting.
Page 34 wrote:As usual the closest attacking model must be moved to contact the closest model in the enemy unit against which the assault was declared. Then remaining models can assault models belonging to other enemy units, as long as they keep following the rules for moving assaulting models.
Bold added for the important bitz
Edits: Emperors Bowels I cannot spell.