Dual solo in pursuit of the Commercial ME is expressly contemplated in the regs. Whereas 61.129(a)(4) (SE) talks about "10 hours of solo flight" including the long x-country and night flight, 61.129(b)(4) (ME) speaks of "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)" including the long x-country and night flight.
So a supervised solo with a qualified instructor is perfectly acceptable for the commercial ME.
Separately, 61.51(d) provides that "[e]xcept for a student pilot performing the duties of pilot in command of an airship requiring more than one pilot flight crewmember, a pilot may log as solo flight time only that flight time when the pilot is the sole occupant of the aircraft." Thus, a pilot would be appropriately disqualified if he took passengers on a required solo cross-country flight. This doesn't really make much sense, once the PPL has been earned, but it does clearly appear to be the rule.
MF