Is it possible to have an RDS MySQL Multi-AZ database instance that spans across two regions?
You can, but the nodes in the other regions are read replicas.
You can create one or more replicas of a given source [MySQL] DB Instance within an AWS Region or across AWS Regions and serve high-volume application read traffic from multiple copies of your data [...]
Multi-AZ Deployments and Read Replicas use different underlying replication technologies suited to their respective purposes. However, you can use them together for reliable, scalable production deployments.
Read replicas have limitations, as you might imagine. You can't write to the replicas (obviously), and there's replication lag, which might lead to data loss if the source database goes down (which is where multi-AZ helps you). The RDS FAQ has some discussion.