only a few cars manufacturers use two cats, infact it's mainly japanese cars that will use two cats. this is because european catalytic converters are a 3 way cat in which one cat deals with the 3 emissions bi products of NOx / HC / CO (oxides of nitrogen, hydrocarbons ie fuel, carbon monoxide). japanese cars use a 2 way cat, which deal with NOx and CO and usually incorporate a small second cat to tackle the HC's ....
I've just spoken to our MOT tester for you, and he's asked if you noticed were the mechanics and staff wearing stirrups when you dealt with them?
the MOT testers handbook states that an exhaust system can only be tested by the VOSA regulations regarding the original build of the exhaust system and tested on the regulations regarding the year or period of production, which offcourse will meet the regulations set out by VOSA for the age of the car. the MOT tester has no way to check wether or not the car came with 1 cat, 2 cats, 3 cats or 10 cats. he can only test what is given to him, to ensure that it meets or exceeds the minimum requirements for emissions
what you want to do is phone VOSA to form an official complaint with the issuing MOT station. remember that they work for VOSA and not the garage in hand, so it's not the actual garages problem.
and to finnish this essay off, offcourse the twingo comes with 1 cat. all petrol powered renaults do, and have done since 1991.
so now you have the word of an former renault RTE to add to your list. REPORT THEM!