Is Harness the Storm's activated ability stackable? So If I have two Harness the Storms on the field and a Titan's Strength in my graveyard and I cast Titan's Strength can I cast two copies of Titan's Strength (assuming I have the mana to pay for two Titan's Strengths) for a total of three Titan's Strengths?
Harness the Storm has a triggered ability, not an activated ability.
If you control multiple copies of Harness the Storm and you cast an instant or sorcery, each one will trigger and allow you to target a card with the same name in your graveyard. However, if there’s only one copy of that card in your graveyard, you’ll only be able to cast that card once from the graveyard. Once you cast it from the graveyard, it stops being the object that the other triggers targeted and so those triggers will be countered on resolution instead of resolving.
For example, you have two copies of Harness the Storm on the battlefield. You cast Titan’s Strength targeting some creature you control. Both enchantments trigger and each trigger targets the lone Titan’s Strength in your graveyard. When the first trigger resolves, you choose to cast Titan’s Strength from your graveyard. This means that you move it to the stack and pay R to cast it, etc. After it resolves, Titan’s Strength goes back to your graveyard as a new object. This means that it’s not the same object that the remaining Harness the Storm trigger targeted, even though it’s the same card. When that other trigger goes to resolve, it no longer has any legal targets, so it’s countered instead of resolving. Finally, the original Titan’s Strength will resolve and end this madness.