"We're a little more prepared but the same excitement is there," the Rascal Flatts guitarist says of baby No. 2. The couple are already parents to 21-month-old son Jagger Donovan. "There's such fulfillment in being a father and it's going to be two-fold now, which is so cool."
Fallon, 35, who is due in September, told Rooney, 34, that she was pregnant when they were staying in the same Los Angeles hotel where he learned they were expecting Jagger.
"It was late January and 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," he 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!' But life just couldn't get better."
The couple are preparing Jagger for the idea of a new sibling, "but he doesn't quite understand yet," Rooney says. "I pray with him every night and we pray for his brother- or sister-to-be."
Mom and dad are eager to prepare as well, and plan to find out the sex of the baby. "Tiff's all about decorating rooms, so she's got to know. She's got to prepare the nest," says Rooney, who is betting on a girl while Fallon suspects it's a boy.
The one element that might prove tricky? Making sure Rooney is present to greet his new son or daughter.
"I'll be on tour this September and the Rascal Flatts machine is not stopping for anybody," he says. "But I have to be there. I'll rent a jet or something if I have to!"