Indie rock lifers unite: The Mountain Goats and Guster bring a double dose of earnest chants and literate jams to Arrowood Farms in Accord. Expect singalongs, seasonal ales, and possibly tears as John Darnielle unspools fan-favorite trauma anthems (hello, “This Year”). Guster’s lush harmonies and sunny hooks make this outdoor show a midsummer dream for elder millennials and indie romantics alike.