There is no standard symbol for this in LaTex. This symbol is often defined as:
You can see it in use on the wikipedia page for Conditional Independence.
If there's such a thing as an 'offical' definition, there's this from the Comprehensive LaTex Symbol List page 106:
Donald Arseneau posted the following \mathpalette-based definition of a probabilistic-independence symbol ⊥⊥ to comp.text.tex in June 2000:
The \independent macro uses \mathpalette to pass the \independenT helper macro both the current math style and the \perp symbol. \independenT typesets \perp in the current math style, moves two math units to
the right, and finally typesets a second—overlapping—copy of \perp, again in the current math style. \rlap, which enables text overlap, is described later on this page.
Frankly, I like the first version better. It might not always render properly in all math fonts, but I can remember it when I need it. Sorry, I don't know how to do this in the equation editor.