It's an interesting idea and one that could potentially uproot the comics industry as we know it. However, I doubt it could ever happen. I believe this because it was actually tried before.
Not many know this, but at one point in the early 80s,
Marvel was very close to buying DC. In fact, it was
so close that Marvel began working on concept art for a re-launched line of DC characters produced by Marvel. It really came close to happening.
A couple years ago, I met Jim Shooter at the New York Comic Con. I actually asked him about this and without hesitation, he said outright that Marvel was ready to pull the trigger. He told me a story about a phone conversation he had with some reps at DC and WB. The feeling was that at the time, WB couldn't turn a decent profit from the comics, but Marvel had apparently figured it out. So the idea was that Marvel would take an unprofitable division (at the time) off their hands for a sum of money and they would do what they did best. According to the lawyers on both sides, they were ready. Only a few signatures would be necessary and it would've happened. Marvel would've bought DC comics.
Then, at the last minute, Marvel was sued by a now-defunct comic company claiming that if they bought DC, they would be violating anti-trust laws. Shooter said the suit sounded frivolous at first, but it was enough of a disruption to cause both sides to drop the deal. They determined that fighting an anti-trust suit would've drained money from both sides and it wouldn't have benefited them in the long run. He did make it clear though. Had that suit NOT been filed, then Marvel would have owned DC. That one lawsuit was the only thing that stopped it.
So it really was that close. And chances are, that was the last opportunity Marvel or DC ever had to join forces. Unless something huge changes in the near future, that'll never happen again.