See the Legendary forums for monthly solo leagues!
From the FAQ:
Captain America, Secret Avenger
Q: Does "Secret Avengers Assemble" give me +2 attack if I play one of the Pet Avengers special sidekicks?
A: No, it only gives a maximum +1 per card. The word "other Avengers Hero" here means "for each Sidekick and each Avengers Hero besides those Sidekicks." Source: Devin