The equation is H2O (l) + H2O(l) ↔ OH- (aq) + H3O+(aq)
We have to know how it could be when the acid donates the H+ and the base accept the H+.
So here we assume that one of H2O is acid and the other H2O in the equation is a base, then the first H2O loses H+ and the second H2O gain the H+.