It should be able to use a property in a singleton from an another singleton, please check the attached files for an example.

SingletonOne.qml:
pragma Singleton
import QtQuick 2.2
import SingletonTest 1.0

QtObject {
property string name: "SingletonOne; uses " + SingletonTwo.name
}

SingletonTwo.qml:
pragma Singleton
import QtQuick 2.2
import SingletonTest 1.0

QtObject {
property string name: "SingletonTwo"
}

Should work.
There can be a circular dependency, because singletonone imports SingletonTest 1.0 and SingletonTwo imports also SingleonTest 1.0, and in order to "Activate" that import SingletonOne needs to be instantiated, which needs to activate the SingleonTest 1.0 import, which needs to...