In my experience, mixing Benzos is like smoking different strains of herb one after another. Yeah, variety is great, but once you've smoked one kind of weed, the subtleties of the next kind will be barely perceivable (IME). The same seems to be true when mixing Benzos. I used to do it to all the time with Xanax and Valium, but to be honest, nowadays I would just prefer to take a little more of one or the other.
But to answer your question, it depends on your tolerance, but you would probably be fine with that combo unless you have no tolerance, in which case you will probably fall asleep quickly and wake up confused. If you're going to go for it, I'd take the Valium first because it takes longer to really kick in. Then, if you feel like you still want it, take the Ativan.