If you have a replication factor of 2 and use CL of Quorum, you will not tolerate failure i.e. if a node goes down, and you only get 1 ack - thats not a majority of responses.
If you deploy across multiple regions, each region is, as you mention, a DC in your Cluster. Each individual DC is a complete replica of all your data i.e. it will hold all the data for your keyspace. If you read/write at a LOCAL_* consistency (eg. LOCAL_ONE, LOCAL_QUORUM) level within each region, then you can tolerate the loss of the other regions.
The number of replicas in each DC/Region and the consistency level you are using to read/write in that DC will determine how much failure you can tolerate. If you are using QUORUM - this is a cross-DC consistency level. It will require a majority of acks from ALL replicas in your cluster in all DCs. If you loose 2 regions then its unlikely that you will be getting a quorum of responses.
Also, its worth remembering that Cassandra can be made aware of the AZ's it is deployed on in the Region and can do its best to ensure replicas of your data are placed in multiple AZs. This will give you even better tolerance to failure.
If this was me and I didnt need to have a strong cross-DC consistency level (like QUORUM). I would have 4 nodes in each region, deployed across each AZ and then a replication factor of 3 in each region. I would then be reading/writing at LOCAL_QUORUM or LOCAL_ONE (preferably). If you go with LOCAL_ONE than you could have fewer replicas in each DC e.g a replication factor of 2 with LOCAL_ONE means you could tolerate the loss of 1 replica.
However, this would be more expensive than what your initially suggesting but (for me) that would be the minimum setup I would need if I wanted to be in multiple regions and tolerate the loss of 2. You could go with 3 nodes in each region if you wanted to really save costs.
