While many people would love a cut and dry yes or no answer to whether or not a woman sleeping with another woman is cheating it's impossible to do so. It is too subjective to the individual agreements and dynamics of each relationship as to what constitutes cheating. Simply put, if you would consider a woman sleeping with a man cheating, the same would apply to two women sleeping together. There is no need for the double standard.
Of course some people argue that perhaps it isn't really cheating since the woman wouldn't be sleeping with another man so psychologically it's different. That's rubbish! If a couple agrees that they are true to each other it makes no difference what gender the person they cheat with belongs to. In the framework of the big picture, in cases like this it isn't about gender, it's about trust.
With that said it is important to realize that not all relationships are strictly monogamous. In some cases people have open long distance relationships in which they give each other permission to have physical relations with other persons under agreed upon guidelines. So long as the guidelines are followed, it is not cheating. There are couples that have an "anything goes" attitude towards his, and others in which some men specifically state they would prefer their female partner only have sex with another woman and frowns upon her having sex with other men.
There are relationships which are simply open in which each partner is free to explore their sexuality with anyone. Usually the only provisions are safe sex and honesty with each other about these trysts. In such an arrangement two women sleeping together would in no way constitute cheating. What about situations when men actually ask their wife/girlfriend to sleep with another woman as part of a threesome or for them to watch? That's not cheating. There are countless arrangements like these briefly discussed in which two women sleeping together would not be cheating.
Considering all of that there is however a far simpler way to determine if two women having sex is cheating, and it is something only those concerned parties know. If the woman in question that has sex with another woman feels like she betrayed her partner or harbors guilt over what she has done then that is as good an indicator as any that she is cheating. Even if she is an an open relationship, she can remain within the boundaries of the agreement, but still cheat emotionally.
With so many variables it is easy to get confused by it all. As much as many people would like to say a woman sleeping with another woman is always cheating, nobody can say that unless they are in fact that woman. What consenting adults do and agree to in regards to their relationship is their own business and so long as they aren't breaking the law or their agreement with each other, it isn't cheating, it isn't wrong, and it isn't for anyone else to judge.