The words " whole time" themselves descibes the nature of the duty a WTD is supposed to perform.
he is entrusted with such duties wherein he is involved for the whole time. hence he can not act / work at any other place at the same time.
A MD is managing the affairs of the company, mainly day to day affairs. - he is also more or less performing duties for the whole time. but the nature of the duties are more of executive and some MD might get time, in some companies, to manage affairs of another co. A person can be MD of two companies at a time.
But the whole time director can not work - rather can not be employed in two companies, at a time.