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.

“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).

“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.