Jackal Familiar

“Can’t attack alone” means Jackal Familiar can’t be declared as an attacker during the declare attackers step unless at least one other creature is also declared as an attacker at that time (either by you or your Two-Headed Giant teammate).

An effect may put Jackal Familiar onto the battlefield attacking even if no other creatures are attacking. That’s because it wasn’t declared as an attacker in this case. Similarly, an effect may put Jackal Familiar onto the battlefield blocking even if no other creatures are blocking.

“Can’t block alone” has a similar meaning. Note that the other blocker(s) doesn’t have to block the same attacker as Jackal Familiar.

Once Jackal Familiar has been legally declared as an attacking or blocking creature, how many other creatures it’s attacking or blocking with no longer matters. For example, if Jackal Familiar and Canyon Minotaur both attack, then Canyon Minotaur leaves the battlefield, Jackal Familiar continues to attack even though it’s now attacking alone.