Mago del Gremio Simic

For the first ability, if the two target creatures aren’t controlled by the same player when the ability resolves, the ability does nothing. The player who controls the two creatures doesn’t have to be the same player who controlled them when the ability was activated, and that player doesn’t have to be Simic Guildmage’s controller.

For the second ability, only the Aura is targeted. When the ability resolves, you choose a permanent to move the Aura onto. It can’t be the permanent the Aura is already attached to, it must be controlled by the player who controls the permanent the Aura is attached to, and it must be able to be enchanted by the Aura. (It doesn’t matter who controls the Aura or who controls Simic Guildmage.) If no such permanent exists, the Aura doesn’t move.

For the first ability, the first target creature doesn’t need to have a +1/+1 counter on it. If it doesn’t, the ability does nothing.