Your unassailable point about Downey and Stallone got me thinking about other possible contenders for the running of most multiple concurrent movie series. They don't all top-line, some are ensemble appearances:
"JOB" series if the long promised sequel to ITALIAN JOB ever materializes
MECHANIC -- possibly in the future as they wimped out and left his character alive. An asterisk.
FAST & FURIOUS
RIDDICK -- sounds like the financing has gotten sorted for this new one.
xXx -- as word is out this may be re-booted
BOURNE (Alternate series thread.)
AVATAR (Upcoming back-to-back sequels)
ALEX CROSS (Upcoming. Conceived as kicking off a series. "I will keep an open mind, I will keep an open mind...")
Oldies & Oddities -- just barely, with perhaps one series slightly over-lapping another: MEL GIBSON