i also got gs150 , kawasaki 125/110, previous owned cd185 twin and i do have manual for few of the bikes i owned but didn't saw where its written.
and pls do not take it as a personal question and let me explain why i asked this question,
lets say u and i both bought same bike on same day from same showroom now we have to ride back to our home u weight around 60kg and let say i am around 110 kg and showroom fellow says sir only ride single no double so we both took bikes back to home so do u think my ride engine will break down because i weight almost double to u?.
what i believe its that this doubling myth is just myth nothing else , when i took my gs150 i asked the showroom guy i will be riding double from the start and he said sure sir its ready to ride and from the day 1 i am riding double and till now no issue. infact engine run far more smoother than my other gs150 who i gave to my younger bro and he believed in riding single till 1k km and when ever he ride my bike he says bro yr bike run smoother than mine.
take care and kind regards