Specifically, if diary Tom Riddle had succeeded, how would he have strengthened the present day Voldemort?
In If diary Tom Riddle had succeeded, would there have been two Voldemorts?, @Pearsonartphoto gives a quote from J.K. Rowling in his answer regarding what would have happened if Tom Riddle had succeeded in fully manifesting from his diary memory form to a full human form.
Q: In Harry Potter and the Chamber of Secrets, what would have happened if Ginny had died and Tom Riddle had escaped the diary?
J.K. Rowling: I can’t answer that fully until all seven books are finished, but it would have strengthened the present-day Voldemort considerably.
JKROWLING.COM - FAQ - Wayback Machine
As well, I've read all seven Harry Potter books many, many times and I haven't been able to ferret out any overt details regarding exactly how the present day Voldemort would've been strengthened by co-existing with diary Tom Riddle.
Specifically, how would the reincarnation of 16-year-old Tom Riddle have strengthened the 67-year-old present-day Voldemort at the time of Chamber of Secrets?
★ I would love an answer from canon, any of J.K. Rowling's interviews, or Pottermore, or an answer constructed within the spirit of canon.
JKR ... making things up– Windle Nov 29 '12 at 22:42