If future Biff goes back to 2015 right after he gives young Biff the almanac, he could get back to the 2015 with Marty and Doc in it.
Jestli se budoucí Biff vrací do 2015 hned po tom, co dá mladému Biffovi almanach, mohl by se vrátit do 2015, kde Marty a doktor jsou.