  We study the entanglement of two disjoint blocks in spin-1/2 chains obtained
by merging solvable models, such as XX and quantum Ising models. We focus on
the universal quantities that can be extracted from the R\'enyi entropies
S_\alpha. The most important information is encoded in some functions denoted
by F_\alpha. We compute F_2 and we show that F_\alpha-1 and F_{v.N.},
corresponding to the von Neumann entropy, can be negative, in contrast to what
observed in all models examined so far. An exact relation between the
entanglement of disjoint subsystems in the XX model and that in a chain
embodying two quantum Ising models is a by-product of our investigations.
