Prolific Lo scored 37 tries last season for Sheffield and starred for Papua New Guinea in the World Cup before moving to the Super League club.
He featured in Castleford’s friendlies against Leeds Rhinos and Newcastle Thunder but faces stiff competition from Super League’s top try-scorer Greg Eden and Greg Minikin at Wheldon Road.
Aston, meanwhile, came through the Eagles academy before signing for Leeds at the end of 2016.
He did not feature there, though, and spent most of the season on loan at Bradford Bulls and Featherstone Rovers before signing a two-yera deal with Castleford in December.
Tigers coach Daryl Powell said: “Both Garry and Cory have been working hard within our environment and this loan period will give them an opportunity to get game time under their belts in the Championship.
“I look forward to seeing them play for Sheffield with a view to them putting pressure on our first-team players in the near future.”
Both are set to come into the Sheffield side for their Championship opener at Dewsbury Rams on Friday and there is an option to extend the loan deal.
Eagles director of rugby Mark Aston - Cory’s father - said: “I’m delighted we’ve been able to bring in Cory and Garry.
“The key thing is they know our systems very well and both were part of this club for some time.
“Garry was absolutely outstanding at the World Cup and had NRL clubs interested in him which shows what he can become if he continues to develop.
“He and Cory are in a great environment at Castleford under the likes of Daryl (Powell) and Shez (Ryan Sheridan) and will be a big boost for us over the next month.”