Simpson, 21, who has been on trial at the Recreation Ground, was released by Bristol City last season after five years at Ashton Gate and had a loan spell with Rovers at the end of the 2003-4 season. He did not play for City but started two games for Rovers and also played 26 games for Tamworth last season.
Senegal-born central defender Sall was released by Kidderminster in the summer, having had loan spells with Oxford United and Nuneaton Borough. He also played for Cinderford Town at the end of the 2003-4 season under Phil Mullen's management.
He played 50 League games for Harriers, many of them alongside ex-Rover Wayne Hatswell, scoring twice.
"Sekani is a good right back, who can also push further on down the right," said Owers.
"I have been chasing him for a number of weeks and I am delighted he is here.
"Abdou was the first player I went after when I got here. I had him in my office four weeks ago but he had to go back to France.
"He is 6ft 4ins and will be a big, strong, physical presence for us at the back."
Both players may feature at Shortwood tonight in Rovers' second friendly of the season, although Owers is again using the game to run the rule over a number of triallists.
It's not known if Terry Brown was interested in signing Simpson but he did appear to be one of the better trialists on show in the matches so far.
With thanks to The Citizen.