Two separate schema imports specify the same target namespace, which is not permitted. For example:

import schema "http://datypic.com/strings"
               at "http://datypic.com/strings/str.xsd";
import schema "http://datypic.com/strings"
               at  "http://datypic.com/strings/str2.xsd";

Instead, you can specify multiple schema locations for the same target namespace in a single import, using commas to separate them. For example:

import schema "http://datypic.com/strings"
               at "http://datypic.com/strings/str.xsd",
                  "http://datypic.com/strings/str2.xsd";

However, the semantics of this are somewhat implementation-defined because schema locations are just hints. A safer option is to create a schema document that includes the two other schema documents, and import that.