  We present the performance of multiplexed XY resistive Micromegas detectors
tested in the CERN SPS 100 GeV/c electron beam at intensities up to 3.3
$\times$ 10$^5$ e$^- $/(s$\cdot$cm$^2$). So far, all studies with multiplexed
Micromegas have only been reported for tests with radioactive sources and
cosmic rays. The use of multiplexed modules in high intensity environments was
not explored due to the effect of ambiguities in the reconstruction of the hit
point caused by the multiplexing feature. At the beam intensities analysed in
this work and with a multiplexing factor of 5, more than 50% level of ambiguity
is introduced. Our results prove that by using the additional information of
cluster size and integrated charge from the signal clusters induced on the XY
strips, the ambiguities can be reduced to a level below 2%. The tested
detectors are used in the CERN NA64 experiment for tracking the incoming
particles bending in a magnetic field in order to reconstruct their momentum.
The average hit detection efficiency of each module was found to be $\sim$ 96%
at the highest beam intensities. By using four modules a tracking resolution of
1.1% was obtained with $\sim$ 85% combined tracking efficiency.
