  We present the experimental realization of a continuous dynamical decoupling
scheme which suppresses leading frequency shifts in a multi-ion frequency
reference based on $^{40}\mathrm{Ca}^+$. By near-resonant magnetic coupling of
the $^2\mathrm{S}_{1/2}$ and $^2\mathrm{D}_{5/2}$ Zeeman sub-levels using
radio-frequency dressing fields, engineered transitions with reduced
sensitivity to magnetic-field fluctuations are obtained. A second stage detuned
dressing field reduces the influence of amplitude noise in the first stage
driving fields and decreases 2\textsuperscript{nd}-rank tensor shifts, such as
the electric quadrupole shift. Suppression of the quadratic dependence of the
quadrupole shift to $3(2)\,\text{mHz}/\mu m^2$ and coherence times of
$290(20)\,\text{ms}$ on the optical transition are demonstrated even within a
laboratory environment with significant magnetic field noise. Besides removing
inhomogeneous line shifts in multi-ion clocks, the demonstrated dynamical
decoupling technique may find applications in quantum computing and simulation
with trapped ions by a tailored design of decoherence-free subspaces.
