Atherton, a lock who last played for South Africa in their Tri-Nations defeat by New Zealand in Cape Town in August, confirmed he had been approached by both Richmond and last season's champions Bath.
Van der Westhuizen, Natal's leading try-scorer of all time with 81, was believed to have attracted the interest of Richmond and as well as Harlequins.
Meanwhile, the South African Sports Minister, Steve Tshwete, accused the country's rugby union chief Louis Luyt of lacking sensitivity towards a failed black challenge for control of the Transvaal Rugby Union.
"I am not happy and I know that many people are not happy," Tshwete said. "If we fail to reconcile the black and white contingents in the South African Rugby Football Union then rugby has no business here."
Tshwete was reacting to reports that Luyt had ridiculed black rugby official Brian van Rooyen who made a failed bid to unseat him as president of the TRU on Monday.Reuse content