Demers, 29, had nine goals and 19 assists in 81 regular-season games for the Panthers last season. He has 37 goals and 134 assists in 504 career games with San Jose, Dallas and Florida.
McGinn, also 29, had nine goals and eight assists in 72 games for the Coyotes last season. He has 100 goals and 84 assists in 522 games with San Jose, Colorado, Buffalo, Anaheim and Arizona.
"We are very pleased to add Jason to our blue-line corps," Coyotes general manager John Chayka said in a team release. "Jason is a skilled right-handed shot defenseman who moves the puck well and has good offensive ability. He will be a great addition to our team."