Can you use two Seals of Unity on the field to pay for the power cost of Harnessed Dragon, given that Seals have the reaction keyword and abilities that add resources can't be reacted to?

Ruling: Yes, you can use both Seals of Unity to pay for the power cost of Harnessed Dragon.

Nuances:

• The restriction that "abilities that add resources can't be reacted to" does not prevent using Seals with the reaction keyword to pay costs
• The game allows you to tap runes or recycle them to keep them available for later during the turn, so you can use Seals even without playing cards directly after
• Seals of Unity add power (the colored symbols), not energy cost (which would be denoted with circled numbers)