is it legal to close a long position in another account?

[loufah]

Quote from cstfx:

You cannot be long and short the same security in the same account. You will be flat.

I've been long and short simultaneously a couple times when I accidentally clicked on "short" rather than "sell" to close out a long position. It was like that for a couple days, until I noticed. Easily fixed - called the broker and they flattened the position.