spacemonaut wrote:
It appears at the bottom of the rulings list. I still don't exactly understand why it sums both while Command Beacon returns only one
Well, Command Beacons says "your commander", in a singular context. While either of your partners qualify as "your commander", you have 2, but as MRHblue pointed out, it's a 1 for 1 exchange the way the card is worded. So you pick one card that fits the criteria.
By the same token, each time you cast either commander, you are "casting your commander", so the commander storm count increases.