Different benzos have different properties (anxiolytic, hypnotic, anticonvulsant, muscle relaxant, and amnesic) at different ratios so usually I find that combining benzos is better than them on their own, since the properties other than amnesic are (usually) all desirable. So synergy is definitely present. However, some benzos are just crappier than others, and I will rather take 2mg of alprazolam than, say, 1mg of alprazolam and the equivalent of that in oxazepam (which would be 40-60mg depending on the source).
But generally, I feel that taking different benzos together is definitely more pronounced than the benzos by themselves. Hypnotic benzos complement each other nicely - just had nitrazepam and temazepam together yesterday. As for Valium and Xanax, I'd take the Valium first, and as soon as you're feeling it good, pop the Xanax for a further "peak" if you will (not that benzos are all that intense in the way of "peaking").
Alcohol, I find, is a nice potentiator in SMALL amounts, so I'll drink 2-3 beers and then take a little less benzos. Saves pills.