Attributes not valid when referencing another schema in the XSD

I have two schemas:

<schema xmlns:foo="urn:x:foo:1.0" xmlns:bar="urn:x:bar:1.0"
        targetNamespace="urn:x:foo:1.0"
        version="1.0"
        xmlns="http://www.w3.org/2001/XMLSchema">
    <element name="bar" type="bar:bar" />
</schema>

and

<schema xmlns:bar="urn:x:bar:1.0"
        targetNamespace="urn:x:bar:1.0"
        version="1.0"
        xmlns="http://www.w3.org/2001/XMLSchema">
    <complexType name="bar">
        <attribute name="goo" type="string" />
    </complexType>
    <element name="bar" type="bar:bar"/>
</schema>

When editing XML in IntelliJ 15 (EAP), this XML validates well:

<bar goo="xxx" xmlns="urn:x:bar:1.0" />

but this cannot find attribute goo (while the element bar itself is fine):

<bar goo="xxx" xmlns="urn:x:foo:1.0" />

I've tried to add <import namespace="urn:x:bar:1.0"/> to foo.xsd but without success.

Is this a bug in IntelliJ or am I missing something? Would you recommend any (preferably online) XML validator capable of handling multiple XSD files?