To my understanding, they're not exactly the same - since the design of each GPU is controlled and optimized by each maker. But as far as what runs where, the reasons are more Marketing and Licencing driven than any real technical differentination in how they two use the PCi buss. Though someone more knowledgeable on the topic would need to speak on the exact detailia, nVidia drivers (used to?) look for a chip that identifies a given motherboard as 'nVidia', and won't work without that chip. This is the means by which nVidia controlled/enforced their "property".
So - It used to be the case that you had to have an nVidia motherboard in order to use SLI, because that is how nVidia Means For It To Be Played(sic). And if the motherboard was not nVidia, then SLI would not work. Crossfire didn't have such a limitation, and their (independent from AMD) past meant that Crossfire worked on both AMD and Intel based boards because ATi made damned sure they had the means to fight their nVidia competitor. Presumably due to contractual obligations, that continued after AMD bought ATi. This meant SLI was nVidia only, and Crossfire was everything else. And why you could have an AMD processor with SLI, so long as it was an nVidia~Licenced chipset.
Now... Queue Intel's introduction of the i7 processor and chipset(s)...
Enter the dreaded "Patent and Contract Lawyers":
Intel took the position that nVidia were only licenced to use Intel processors/sockets/technologies up to LGA 775, but NOT LICENCED for LGA 1366 and forward. i.e. "Non~FSB based Technologies". Understand that this was done against a background of Jen-Hsun Huang and his buddies at nVidia consistently, and over a long time period (still counting, actually) telling the world that they were taking over and that they were going to kill Intel. The "Right~ness" of Intel's reaction to nVidia - and whether it was Intel's fault or nVidia's - is debatable. (not to mention moot) But the courts upheld Intel's position and result was nVidia were totally, legally, and royally screwed out of using Intel's latest/greatest processors on nVidia motherboards.
That's right: No i7's on nVidia, though they could continue to design, manufacture, market, and sell LGA775 based mobos. (gee... thanks...)
Eventually - after nVidia agreed to swallow what Intel presented to them - the end result of Intel's "Let's Bend Mr Huang Over The Table And Insert Our... Position..." set of "Negotiating Tactics" was that Intel got the right to use SLI on Intel based motherboards, and other manufacturers also granted permission to do the same so long as they paid nVidia licencing fees for the SLI brand.
{Edit, with an apology for not making this clear earlier}In return nVidia got the right to design, manufacture, and market motherboards based on the newer Intel chipsets and sockets. But in the end, that didn't matter since {/Edit} a little after this, nVidia gave up on making motherboards and chipsets altogether and closed/sold off that arm of their business.
...and there you have it. Intel based motherboards can offer both sets of technology, provided the board's maker pays nVidia for the right to use "SLI". AMD motherboards obviously come with the right to use Crossfire, and some of the makers choose to pay nVidia for SLI rights as well.
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