Depends on the design of the fitting, the basic type with 2 completely separate sets of wirewound ballast and starter circuitry usually can. Types which use a single box electronic ballast for both tubes usually can't.
If you can post a picture showing all the internals of your fitting, you may get a more helpful answer...
Whats the point, is it running costs.
Running two tubes on an electronic ballast is proberly cheaper to run than one tube of a starter magnetic /choke fitting, you are unlikely to find a twin fitting with 2 electronic ballasts fitted and most wholesalers wont know and proberly wont let you open them to find out, even if they do exist
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