avast! uses only its own engine.
Multi-engine AV, however, is not such a miracle as you might think. First, there's no guarantee that 2 engines detect all the viruses (they don't, of course - you might have a new piece of malware that none of the engines on Jotti / VirusTotal detect, but it's still malware). Actually, there's no guarantee that there's any improvement if you add the second engine (what if the detection of the second engine is a subset of the first one?)
In reality, the improvement in going from one engine to two will be rather small, IMHO.
Appart from the licensing stuff (you will have to pay more for 2 engines than for one, because the other vendor wants some money, too), there are other disadvantages as well - the false positives sum up from the two engines, the scanning is slower (because you actually scan everything twice)...
