There is no requirment by the FARS to solo the twin ever before you get your rating. For an add-on rating there is no time requirment except for the 3 hours in 60 days. for an initial commercial student in the twin it states 61.129 (B) (4): "10 hours of solo flight time in a multiengine airplane or 10 hours of flight time performing the duties of pilot in command in a multiengine airplane with an authorized instructor (either of which may be credited towards the flight time requirement in paragraph (b)(2) of this section), on the areas of operation listed in §61.127(b)(2) of this part that includes at least"
and il answer your next question. How can a person without a multi rating log PIC? the MEI must give them an endorsment to act as PIC of an aircraft they do not hold a rating in. the endorsment is in the Advisory circular