MongoDB replication to local hard drive
Replication in MongoDb is a simple process to setup and I configured it locally during an online course I completed via the 10gen website, obviously this will fail over without an issue to your other instance but depending upon what fails on your machine will it be of any use!

I'd advise working your way through the free course they have it as it will solve 90% of the queries you will have with evaluating the product (replica sets are part of this)

We had it my old company and did run it on several machines for full redundancy and automatic failover to another node, you'll need a witness too.