  In disordered systems, wave functions in the Schr\"{o}dinger equation may
exhibit a transition from the extended phase to the localized phase, in which
the states at the boundaries or mobility edges may exhibit multifractality.
Meanwhile, the Critical Phase (CP), where all states exhibit multifractal
structures, has also attracted much attention in the past decades. However, a
generic way to construct the CP on demand still remains elusive. Here, a
general approach for this phase is presented using two coupled quasiperiodic
chains, where the chains are chosen so that before coupling one of them has
extended states while the other one has localized states. We demonstrate the
existence of CP in the overlapped spectra in the presence of inter-chain
coupling using fractal dimension and minimal scaling index based on
multifractal analysis. Then we examine the generality of this physics by
changing the forms of inter-chain coupling and quasiperiodic potential, where
the CP also emerges in the overlapped spectra. We account for the emergence of
this phase as a result of effective unbounded potential, which yields singular
continuous spectra and excludes the extended states in the overlapped regimes.
Finally, the realization of this CP in the continuous model using ultracold
atoms with bichromatic incommensurate optical lattice is also discussed. Due to
the tunability of the two chains, this work provides a general approach to
realizing the CP in a tunable way. This approach may have wide applications in
the experimental detection of CP and can be generalized to much more intriguing
physics in the presence of interaction for the many-body CP.
