Wicked Slumber can target creatures that are already tapped.

If you choose two target creatures, but one of them is an illegal target as Wicked Slumber resolves, the illegal target will be unaffected. You'll put both stun counters on the remaining legal target. If both targets become illegal, Wicked Slumber won't resolve at all.

The two stun counters can end up on the same creature, or they can each end up on a different one of the target creatures.

Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.

When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.

Tapping an untapped creature that's attacking or blocking to convoke a spell won't cause that creature to stop attacking or blocking.

Tapping a multicolored creature using convoke will pay for or one mana of your choice of any of that creature's colors.

You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.

If a creature you control has a mana ability with in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.