I would like to install a Default instance with SQL Server 2008 R2 RTM and I want to install another named instance with SQL Server 2008 R2 SP2. Is this possible?
Yes, you can choose instance to apply service pack.
Instance components along with shared components between side by side instances will be upgraded to higher service pack.
Sql server has two types of component\features.
1. Instance specific features
2. Shared Features
When you upgrade one component with a given instance name to a new version or service pack, you must upgrade all components of that instance.
Yes, but you have to understand that there are certain shared components. See Work with Multiple Versions and Instances of SQL Server.