Alan Mitchell, 34, from Cambuslang, and Gary Mitchell, 20, from Shettleston, Glasgow, carried out the raid at Eric N. Smith jewellers in Ayr Road, Newton Mearns, on April 13, with another unknown man.
The jury at the High Court in Glasgow was played CCTV footage which showed the masked robbers walking along Ayr Road with one of them carrying a large lump of concrete on his shoulder,
This stone was used to hold open the shop door.
One of the robbers threatened the owners Eric Smith and his wife Yvonne with a machete while the other two used hammers to smash display cases containing Rolex watches.
Thirty-eight watches worth £400,000 were taken and a further £210,000 worth of damage was caused to other Rolexes.
Mrs Smith, 52, said: "The first thing I was aware of was a really loud bang. It was like a gun going off.
"I ran towards where the noise was coming from. There were three men coming into the shop with masks and carrying a machete and two hammers.
"They kept shouting 'Get back, get back.'"
In court, Mrs Smith identified the Mitchell cousins as the two men she saw in the shop.
She said she thought Alan Mitchell was the leader and Gary Mitchell: the one wielding the machete.
Both men denied having anything to do with the robbery. Alan Mitchell claimed he was getting a quote for work from a plasterer at the time and his cousin claimed he was at a football match.
Advocate depute Sheena Fraser, prosecuting, told the court that both men have previous convictions.
Judge Lord Jones deferred sentence on both until November 13 at the High Court in Edinburgh.