Bell and Templeton were teammates as Mark Warburton's Rangers won the Championship last season, but left after their Ibrox contracts expired.
The pair were teammates at Ibrox until the start of this season, when both were released and Bell linked up with the Terrors.
David Templeton in action for Rangers
And Templeton is currently training with the Championship side, as he looks to earn his first contract since leaving Rangers.
The winger has endured a torrid time with injuries, which have sidelined him since August 2015.
And, according to McKinnon, Templeton must get himself in shape if his trial spell is to lead to something more long-term.
"Cammy Bell asked if David could train and I said no problem," revealed the Dundee United boss. "They’re friends from Rangers and travel together.
Former Ibrox shot-stopper Cammy Bell applauds
"You can see he’s a good player but he is still a bit away from fitness because of his weight. He needs to get himself into shape, so that’s what we’re doing."
Rangers are said to have paid £700,000 to land Templeton back in 2012, after the 28-year-old impressed for Hearts.
In total, he appeared 88 times for the Gers, scoring 27 goals and helping his hometown team to three promotions.
Would Templeton still be a Glasgow Rangers player if it weren't for his injuries?