Without MongoDB and clustering there are a lot of good options to have two author instances which operate in parallel.
In 6.0 you can still run CRX2 and you could do the old style clustering. The big down side is that you won't have an upgrade path - in 6.1 CRX2 support is no longer available.
You can use job off loading to move workflow processing or other job based processes to run on another author which removes load. You still have to do your authoring on one instance but if you have lots of image rendition creation work or other batch processing it can help.
The only other viable solution is some sort of sharding where you edit half your content in Author 1 and the other half in author 2. That's problematic however because it is really difficult to enforce systematically and its easy end up with overwrites and conflicts. It would stay away from a scenario like that.
If you have a situation where you really legitimately requires an active - active scenario you need to take another look at MongoMK - its the only good solution in my opinion.
