I have seen from many different sources the thought process that you can have your Alchemical Homunculus or your Steel Defender use your spell storing item. This would let them take up concentration for an additional spell.
However the more I look into it the less I think you can. I wanted to get clarification from you folks.
Spell Storing item:
While holding the object, a creature can take an action to produce the spell’s effect from it, using your spellcasting ability modifier. If the spell requires concentration, the creature must concentrate.
Alchemical Homunculus:
In combat, the homunculus shares your initiative count, but it takes its turn immediately after yours. It can move and use its reaction on its own, but the only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take the action in its stat block or the Dash, Disengage, Help, Hide, or Search action.
Steel Defender:
In combat, the steel defender shares your initiative count, but it takes its turn immediately after yours. It can move and use its reaction on its own, but the only action it takes on its turn is the Dodge action, unless you take a bonus action on your turn to command it to take one of the actions in its stat block or the Dash, Disengage, Help, Hide, or Search action.
I added emphasis on only. This completely negates the ability to use the Spell Storing Item in combat doesn’t it?
Best Answer
After errata, those creatures can use a Spell Storing Item
As part of the errata accompanying the reprint in Tasha's Cauldron of Everything (pre-errata answer below), these command rule for these creatures where updated to say:
This enables them to use any action including the Use an Object action (as clarified by the SAC) used by the Spell Storing Item to produce the spell effect.
Before errata, those creatures can't use a Spell Storing Item
The key here is that the Spell Storing Item requires a creature to use a special (unnamed/Use a Magic Item†) action to produce the spell's effects. Both these creatures are limited to taking the Dodge action (if not given a command), taking one of the listed generic actions, or taking an action in it's stat block (where the Spell Storing Item is not written).
The override here (ie. DM ruling/houserule) would be deciding that a creature/monster having an object would add any relevant special actions to its stat block (as would presumably be done when the DM customizes a monster), but there is no rule covering whether this happens dynamically or not. Without such a ruling they can't use the item.
†: This action is named as Use a Magic Item in the Essentials Kit and in many discussions, but not in the Player's Handbook nor the Dungeon Master's Guide. Regardless it is distinct from Use Object and Cast a Spell, not that it happens to be relevant here.