  We investigate the competition between pair entanglement of two spin qubits
in double quantum dots attached to leads with various topologies and the
separate entanglement of each spin with nearby electrodes. Universal behavior
of entanglement is demonstrated in dependence on the mutual interactions
between the spin qubits, the coupling to their environment, temperature and
magnetic field. As a consequence of quantum phase transition an abrupt switch
between fully entangled and unentangled states takes place when the dots are
coupled in parallel.
