  We report the realization of an advanced technique for measuring relative
length changes $\Delta L/L$ of mm-sized samples under control of temperature
($T$) and helium-gas pressure ($P$). The system, which is an extension of the
apparatus described in Manna et al., Rev. Sci. Instrum. 83, 085111 (2012),
consists of two $^4$He-bath cryostats each of which houses a pressure cell and
a capacitive dilatometer. The interconnection of the pressure cells, the
temperature of which can be controlled individually, opens up various modes of
operation to perform measurements of $\Delta L/L$ under variation of
temperature and pressure. Special features of this apparatus include the
possibilities (1) to increase the pressure to values much in excess of the
external pressure reservoir, (2) to substantially improve the pressure
stability during temperature sweeps, (3) to enable continuous pressure sweeps
both with decreasing and increasing pressure, and (4) to simultaneously measure
the dielectric constant of the pressure-transmitting medium helium,
$\varepsilon_{\mathrm{r}}^{\mathrm{He}}(T,P)$, along the same $T$-$P$
trajectory as used for taking the $\Delta L(T,P)/L$ data. The performance of
the setup is demonstrated by measurements of relative length changes $(\Delta
L/L)_T$ at $T=180\,\mathrm{K}$ of single crystalline NaCl upon continuously
varying the pressure in the range $6\,\mathrm{MPa}\leq P \leq
40\,\mathrm{MPa}$.
