Part using twin flourescent fittings - OK or not ?

[ajh]

Hi guys ,
I have a number of Philips downlights each fitted with two PL-C 13 watt flourescent lamps ( i.e. twin fittings). With both the lamps lit the fittings are really too bright for the area they are installed in and I would like to reduce the light intensity. Questions :-
a) The fittings have twin ballasts and twin PL-C lamps . Would it damage or shorten the life of the fittings if I just removed one of the lamps ?. - i.e. just ran them with one PL-C lamp installed .

b) If that is not recommended could I just install lower wattage PL-C lamps :- i.e. 2 x 11 watt or better 2 x 7 watt etc.

There don't appear to be any starters in the fittings just the two ballasts - one for each lamp .

Hoping someone can help me as I don't want to damage the fittings.
Thanks in advance