Rashford and Martial have combined to score 10 goals already this season. It has helped United start the new campaign with 33 goals in just 11 games in all competitions.
But Mourinho has rarely paired Rashford and Martial together in attack, starting the pair in the same team just once this season in the 4-1 win over Burton Albion in the Carabao Cup.
"The battle between Marcus Rashford and Antony Martial has been great to follow," Giggs told Sky Sports.
"I could definitely see them in the same team, with Marcus being more of a natural centre-forward whereas Martial is more of a left-sided forward but can also play centrally."
Martial, who arrived at Old Trafford from Monaco for £36 million, was voted Europe's best young player in 2015.
Rashford, meanwhile, has been nominated for this year's Golden Boy award.
But Giggs insists the pair, who scored 19 goals between them last season, will need to up their goal output before they can be considered world class.
He added: "They've got incredible pace so they are extremely dangerous, although to be considered top players they need to be scoring 20 goals a season."