  We propose a technique for accurate, flexible and robust generation of
arbitrary coherent superpositions of two quantum states. It uses a sequence of
two adiabatic pulses split by a phase jump serving as a control parameter. Each
pulse has a chirped detuning, which induces a half crossing, and acts as a
$\pi/2$ pulse in the adiabatic regime. The phase jump is imprinted onto the
population ratio of the created superposition state. Of the various possible
relations between the two pulses, we select the case when the Rabi frequency
and the detuning of the second pulse are mirror images of those of the first
pulse, and the two detunings have opposite signs. Then the mixing angle of the
superposition state depends on the phase jump only. For other arrangements, the
superposition mixing angle is shifted by the dynamic phases of the propagators,
which makes these cases suitable for state tomography. This twin setup comes
along with the advantage that it reduces the error $\epsilon$ of each
individual pulse down to $4\epsilon^2$ overall. Therefore, the proposed
technique combines the benefits of robustness stemming from adiabatic evolution
with accuracy generated by the twin-pulse error suppression, and flexibility of
the superposition state controlled by the phase jump $\phi$. In addition, we
present a simple exactly soluble trigonometric model in order to illustrate the
proposed technique. In this model, when the pulse area $A$ increases the
nonadiabatic oscillations are damped as $A^{-1}$ for a single pulse and
$A^{-2}$ for the two-pulse sequence. Finally, the proposed technique is
extended to sequences of $N=2^n$ pulses by concatenating $\pi/2$ sequences and
splitting them by a phase jump, thereby further reducing the nonadiabatic error
$\epsilon$ to $(2\epsilon)^{N}$. This makes the proposed technique suitable for
generating high-fidelity quantum rotation gates.
