Waratahs club captain Dennis and experienced midfielder Horne are now expected to start against The Lions in game five of the 125th anniversary tour.
The news is a second major boost in 24 hours for Waratahs head coach Michael Cheika, who yesterday confirmed that first-choice fly-half Bernard Foley and reserve scrum-half Matt Lucas would also be available for selection after they were given special dispensation for a temporary release from the Australia Sevens squad.
Lions counterpart Warren Gatland also views the news as a positive, with the New Zealander keen for his side to face the strongest possible opposition in the lead up to the first of three Test matches.
"It's an opportunity for those players, if they're not going to be selected for the first Test, to have a run against us and perhaps even go back with a bit of information for the Australian side as well. It's great for the players and it's great for us," said Gatland.
Those sentiments were echoed by Lions No8 Jamie Heaslip, who will come into close contact with Dennis if the 27-year-old starts on the blindside flank for the Tahs as expected.
"We'll look forward to it. We want to be going up against the best players that are available. We want to be constantly challenged and constantly pushed and that's what we're looking for this weekend," added Heaslip, who will pack down alongside skipper Sam Warburton and Tom Croft in the back row.
Dennis and Horne aren't the only Qantas Wallabies to have been given permission to face The Lions before the Brisbane opener, with Deans also releasing Brumbies duo Peter Kimlin and Scott Sio for Tuesday's clash in Canberra.
Lock Kimlin and prop Sio were among the six additions to Deans' squad at the start of the week and are now set to start the final match before the Test series begins.