  We present a reanalysis of the HeII Lyman alpha absorption towards the
quasars HS1700+6416 and HE2347-4342 using new high S/N, optical observations.
An alternative analysis method is applied, which fits the high quality, optical
HI data directly to the HeII spectrum. The results are compared to those
inferred from standard line profile analyses. This new method enables us to
derive redshift scales characterizing the fluctuations of the column density
ratio eta. We find eta changing smoothly with redshift on typical scales of
Delta z ~ 0.01-0.03 corresponding to 8-24 h^-1 Mpc comoving. The real length
scales of variations of the column density ratio might be even larger, since
part of the fluctuations may be caused by noise in the HeII data and by effects
due to the applied method. However, eta variations on small scales of a few Mpc
with an amplitude of about +/- 1.5 dex cannot be ruled out completely. The data
shows an apparent correlation between low eta regions and the presence of metal
line absorbers, which corresponds to the more general correlation of low eta
and strong HI absorption. Thermal line broadening is suggested as a probable
explanation for this apparent correlation, since both fit methods would
severely underestimate eta for absorbers with log N(HI) > 13 if the line width
was dominated by thermal broadening. Indeed, lines located close to the cut-off
of the b(N) distribution yield lower column density ratios compared to the
whole sample, in particular if high density absorbers are considered. We argue
that the apparent correlation of eta with the strength of the HI absorption is
caused by insufficient consideration of thermal broadened lines by the standard
analysis. As unbiased value of the column density ratio, we find eta ~80 in
agreement with previous estimates.
