mudgal:
The answer to your original question ("...if I already am signed up with NW WP 'dining for miles' can I 'double dip' by entering another credit card with idine.com, thus receiving twice the miles?") is as MIKESILV & cordelli stated, though I wanted to say something regarding the statement "I would expect as they merge all their data files that will go away at some point in the future." (re: getting around the one visit per month rule by registering cards for different programs):
Thankfully, that won't be the case...
The data is already merged -- Dining for Miles was essentially a co-branded offering of Dining a la Card--until Dining a la Card merged w/ Transmedia, to form iDine Prime. iDine is an iDine Prime derivative offering which does not include an annual fee, & allows the choice of airline miles or "loyalty points" in place of cashback rewards.
The airline affiliated dining programs are just specific airline mile earning forms of the iDine program--same customer service center, same everything.
I learned some additional details from a mistake they made w/ my parents' accounts (resulting in their "Flying Free Faster" @ a much slower rate...
) -- I registered my parents' WorldPerks accts & a single credit card each w/ Dining for Miles, & they soon received welcome materials for the US Airways Dividend Miles program -- ?! -- complete w/ US Airways FF #s (& the #s were 'n' & 'n+1' [i.e. clearly seemed to be generated in succession (& automatically, presumably from a "block" of #s)]). My parents didn't have US Airways frequent flyer accts prior to such...
Obvious conclusions = a field indicating airline is part of the iDine database (somewhat of a "duh"), & if the associated FF acct field is empty (when the record is saved, presumably), a FF# gets generated (perhaps?). I called to have the DividendMiles-instead-of-DiningForMiles mistake corrected, & seem to have confirmed my suspicions in the process (per info from the rep w/ whom I spoke) re: airline field & auto-generation of FF#. I think the automatic generation of FF #s is a ridiculous reality--if such a # wasn't provided, why credit one which the acct holder is unaware of--?!...or if the field gets zeroed out by mistake, let it be a "flag" of some kind, & follow up, accordingly. The kicker w/ my parents' program registration situation was that as soon as the rep changed the airline specification to NW, a WorldPerks # that I had previously provided (i.e. during the original phone call) appeared (for one of the accts)--it had been successfully entered during the original call, but ultimately an incorrect specification in the airline field was made (& the relational aspect of Northwest + FF#/WorldPerks# was replaced by US Airways + FF#/DividendMiles#). The other acct didn't even have the WorldPerks # I had previously provided (note to self: if I ever register another card w/ them, call them back & confirm the validity of all relevant entries [including the spelling of names, as they MUST match that of the FF acct as registered w/ the airline (for NW, @ least, though presumably for others, as well]).
The limit of what is possible re: registering credit cards w/ iDine dining programs comes down to the # of credit cards--a credit card can be associated w/ only one of their offerings, period. For example, you could have 3 cards registered w/ iDinePrime (& pay the requisite annual fee, & receive cashback for relevant dining purchases made w/ said cards), 3 other cards registered w/ Dining for Miles, 3 other cards registered w/ Dividend Miles Dining, 3 other cards w/ Delta's program, etc.
The bottom line is that you cannot receive multiple iDine-related benefits for the same credit card. That's it! Other than that, knock yourself out!
Regarding cactuspete's question re: what happens if/when you change the FF# associated w/ a registered card:
Given that a change to the relevant database record is dynamic (& would be reflected in the next sweep re: mileage credits to associated accts), my suspicion was that to ensure miles originally charged for credit w/ the "original" FF acct made it into said acct, one would have to wait for the next sweep (assuming all relevant dining instances are "in the system")(the regular acct sweeps take place monthly [though they can credit accts in between such times (i.e. sending corrections, missing mileage credits, etc.)]). I called a rep to inquire about such, & was informed that that is correct; in fact, iDine's recommendation = waiting 2 months, just to be sure. What I cannot help but think, however, is that in such a circumstance, one could possibly get them to send a transmission over outside of the monthly sweep. The missing mileage credit which I called about months ago was handled in a courteous, expeditious manner; the deposit showed up within a week, & I even received a phone call from a DiningForMiles rep informing me of when the info was to be sent over, & a direct contact # should I have any questions or problems regarding (I was pleased).