The Books Online have a lot of information on this topic.

We've been running ACTIVE/ACTIVE clustering for years and even have a cluster with SQL2005 and SQL2000. It's totally supported by Microsoft.

When running an ACTIVE/ACTIVE cluster what you have to always keep in mind during the design is one day both INSTANCES may indeed run on one node because of a failover, so you must design the cluster and configure SQL Server with that in mind. Will you have enough resources to run on one node? You may need to purchase more RAM to meet the needs of both instances.

As mentioned above, you'll want to set MAX_SERVER_MEMORY to half of total memory for each instance, therefore when failover occurs you're not running into memory pressure between the two instances.