Once your two instances are up and running, I literally followed the Riak documentation at Basic Cluster Setup. It was really that easy. The was one gotcha, though. Make sure that when you choose the IP address to bind (and name) for the nodes in your cluster that you use the EC2 Private IP Address (or DNS name should be fine too.)
This blog is called myNoSQL and it is written by me, Alex Popescu, a software architect with a passion for open source and communities.
It records my readings, learnings, and opinions on NoSQL databases, polyglot persistence, and distributed systems -- subjects that I'm passionate about.
The opinions expressed here are my own, and no other party necessarily agrees with them.
If you feel I'm biased, I probably am.