  Supergiant fast X-ray transients are wind-fed binaries hosting neutron star
accretors, which display a peculiar variability in the X-ray domain. Different
models have been proposed to explain this variability and the strength of the
compact object magnetic field is generally considered a key parameter to
discriminate among possible scenarios. We present here the analysis of two
simultaneous observational campaigns carried out with Swift and NuSTAR
targeting the supergiant fast X-ray transient sources AX J1841.0-0536 and SAX
J1818.6-1703. A detailed spectral analysis is presented for both sources, with
the main goal of hunting for cyclotron resonant scattering features that can
provide a direct measurement of the neutron star magnetic field intensity. AX
J1841.0-0536 was caught during the observational campaign at a relatively low
flux. The source broad-band spectrum was featureless and could be well
described by using a combination of a hot blackbody and a power-law component
with no measurable cut-off energy. In the case of SAX J1818.6-1703, the
broad-band spectrum presented a relatively complex curvature which could be
described by an absorbed cut-off power-law (including both a cut-off and a
folding energy) and featured a prominent edge at $\sim$7 keV, compatible with
being associated to the presence of a "screen" of neutral material partly
obscuring the X-ray source. The fit to the broad-band spectrum also required
the addition of a moderately broad ($\sim$1.6 keV) feature centered at $\sim$14
keV. If interpreted as a cyclotron resonant scattering feature, our results
would indicate for SAX J1818.6-1703 a relatively low magnetized neutron star
($\sim$1.2$\times$10$^{12}$ G).
