Concatenate multiple strings in XML?

No I don't think you can concatenate.

<string name="aaa">aaa</string>
<string name="bbb">bbb @string/aaa</string>

Output - bbb @string/aaa

If you do,

<string name="aaa">aaa</string>
<string name="bbb">@string/aaa bbb</string>  -> This won't work it
                                                      will give compilation error

Because here it will search for a String with reference @string/aaa bbb which does not exists.

Problem in your case was, you where using @strings/aaa which should be @string/aaa

No, you can't concatenate strings in XML but you can define XML resources.

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE resources [
  <!ENTITY appname "MyAppName">
  <!ENTITY author "MrGreen">
]>

<resources>
    <string name="app_name">&appname;</string>
    <string name="description">The &appname; app was created by &author;</string>
</resources>

The original answer was posted here.

In XML only this is not possible but using the java code you can use the String.format() method.

<string name="aaa">aaa</string>
<string name="bbb">bbb %1$s</string>

In java code

String format = res.getString(R.string.bbb);
String title = String.format(format, res.getString(R.string.aaa));

So title will be a full string after concatenation of two strings.