Joe Don Rooney, Tiffany FallonJagger Rooney is going to be a big brother!

Rascal Flatts guitarist Joe Don Rooney and his wife, former Playboy Playmate Tiffany Fallon, are expecting their second child in September. The couple, who were married in 2006, are already parents to Jagger Donovan, who will celebrate his second birthday in May.

"We're a little more prepared but the same excitement is there," Joe Don tells PEOPLE of getting ready for baby No. 2. "There's such fulfillment in being a father and it's going to be two-fold now, which is so cool."

Tiffany broke the good news to her hubby when the two were staying in the same Los Angeles hotel where she told him she was pregnant with their first baby.

"We were out by the pool chilling and she said, 'I have something to show you,' and she opened her purse and pulled out the stick with the positive sign," Joe Don says. "I was like 'Wow! We're going to have to stop coming to this hotel. We'll have 16 of these things before long!'"

Joe Don says he has a feeling the baby is a girl, while Tiffany is guessing it's another boy. They plan to find out the sex as soon as they can. "Tiff's all about decorating rooms, so she's got to know ... She's got to prepare the nest!" says Joe Don, who adds, "Life just couldn't get better."