Can I obtain a new voltage source from outputs of two cascaded linear voltage regulators in practice?
NO you cannot!

What most people don't appreciate, apparently even some people with a lot of experience is that a voltage regulator is not the same as an ideal voltage source.  The 78XX series can only supply current.  In the topology you are suggesting the lower 7805 must be able to sink current into it's output.

What will happen is that once you device starts to pull current the 7805 will see it's output pulled high, it will attempt to control this condition by reducing the control signal on it's pass transistor, shutting it down a little bit, this shows less of a load on the bottom side so the voltage bumps up more etc. etc. until the 7805 output is at the 12 V rail and your 7V device is shut off.

You have to have a power supply that can both source and sink current into it's out put.  So a Op-amp style with a complementary output structure.

Another alternative is to use a 7905 with it's bottom rail being your implied ground (18 V negative ).  7905's are design to sink current into a negative rail.