I was there a year ago and at the New Bond St boutique, they enforced it for the 2 black caviar WOCs I was buying (one for me, one for a friend), so I had to have my roommate purchase the second WOC. When I brought it up at Heathrow when I was leaving, the SA there told me she didn't really care and would still ring me up for more SLGs (cardholder).
...so I think it's boutique/SA dependent and applies to all goods if you're planning on claiming the VAT back or doing another duty free option (ie. Heathrow) - when they quoted the rule at me, it was "Only one bag per passport if you're claiming VAT back". You might be able to get away with it at Harrods since it's in Harrods and you would claim your VAT back through Harrods - but haven't tested it. Would be curious to see if anyone has done this since I'm planning another trip there in the fall.