The two idioms are used interchangably:
Marcia will attend, as long as she can sit next to Greg.
Marcia will attend, so long as she can sit next to Greg.
Another one that is used identically:
Marcia will attend, provided that she can sit next to Greg.
From these examples, you can see that the "long" doesn't have to refer to length or number. "As long as..." roughly means "but only if..."
Emily Sledge | Manhattan GMAT Instructor | St. Louis
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