Should classes from the same namespace be kept in the same assembly?

Question

For example, ISerializable and the Serializable Attribute are both in the System.Runtime.Serialization namespace, but not the assembly of the same name. On the other hand, DataContract attributes are in the namespace/assembly System.Runtime.Serialization.

This causes confusion when a class can have using System.Runtime.Serialization but still not have reference to the System.Runtime.Serialization assembly, meaning DataContract cannot be found.

Should this be avoided in practice, or is it common for namespaces to be split over multiple assemblies? What other issues should one be careful of when doing this?