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Well like I said it depends on what keywords you target... So if those keywords are ranking high on BING but not providing good traffic, then it is a waste of time. Until you get top positions in BING, then you cannot know for sure. Here is BING's keyword tool. In the last 6 months the word "Google" was searched in bing 1.3 billion times.

Hi Anthony, thanks for your answer. Sorry, but this is a duplicate question. After I posted it first time, SEOmoz still reported I didn't post any questions, ever. So I went ahead and typed it again. Your answer is valuable, so I will mark it as a good answer.

Depends how unique the pages are. If the "landing pages" only have minor differences then you could well be penalizsd a may want to canonicalise them back together.  Certainly if you have the exact same content on both of those sites separately then you'd run the risk of duplicate content. 25k landing pages, to me, may also be thought of as spammy as you'll have more landing pages than anything else.