There's no question (if you ask the pilots involved) that the RIO or WSO were absolutely an essential "part" of the victory(ies).
Cunningham, Ritchie and other two-seater pilot aces all have willingly 'shared' credit over the years on TV and in print with their guy in back. F-4 backseaters Chuck DeBellevue (USAF), Jeffrey Feinstein (USAF), and Willy Driscoll (USN) all "made ace", at least that's what they were and are called by many, and have been featured as such prominently in special interest pubs and television programs.
I believe that DeBellevue actually came back and went through UPT and ended up flying F-4s and F-15s, but don't quote me on that.
There are many other cases of backseaters like Steve Croker (2 kills with Robin Olds) who didn't necessarily "make ace" in the 5-kill sense, but for scoring a victory they received the same "Phantom Club" certificates from McDonnell Douglas as the pilots did. Croker's is signed by Olds himself, and it's one his most cherished possessions (I saw it in Lt Gen Croker's office when he was 8AF/CC). Croker and Olds, BTW, used the Air Force Museum's Phantom #829 for their kills (their actual helmets are, or were, on that plane).
Croker case brings up another wrinkle: in the early days some of the backseaters, like Croker, were actually rated qualified-in-the-Phantom pilots, and are officially listed as "copilot", though their duties at the time in question were of course identical to a RIO or WSO, though I think putting "pilots in the backseat" was mainly an AF thing until they realized they didn't "need" pilots back there in combat as they originally thought.
The "last word" in regards to ace matters, at least according to the
American Fighter Aces Association, resides with the peerless Frank Olynyk. Vol. 10 (USA - Post WWII) of his "aerial victories" series lists all of the WSO and RIO victories, but the 5-kill (or more) backseater types are NOT considered "aces", by Olynyk, i.e., their victories are detailed, but each of their final scorecards is 0/0/0 - no confirmed, no probable, no damaged credits.
EDIT: I just checked the Air Force Historical Research Agency site,
http://afhra.maxwell.af.mil/avc/avc.asp, and the Air Force backseaters are all credited with their victories. As to your original question, "Are they Aces?", well, 5 or more official credits generally makes you an ace, but I think the best way to refer to them when it comes to the Pilot-WSO/RIO team is "Pilot Ace" or "Backseater Ace". In any case, as indicated, the AF
officially credits them with the kills, so there you go.
Wade