  Accurate forecasting of the arrival time and arrival speed of coronal mass
ejections (CMEs) is a unsolved problem in space weather research. In this
study, a comparison of the predicted arrival times and speeds for each CME
based, independently, on the inputs from the two STEREO vantage points is
carried out. We perform hindcasts using ELlipse Evolution model based on
Heliospheric Imager observations (ELEvoHI) ensemble modelling. An estimate of
the ambient solar wind conditions is obtained by the
Wang-Sheeley-Arge/Heliospheric Upwind eXtrapolation (WSA/HUX) model combination
that serves as input to ELEvoHI. We carefully select 12 CMEs between February
2010 and July 2012 that show clear signatures in both STEREO-A and STEREO-B HI
time-elongation maps, that propagate close to the ecliptic plane, and that have
corresponding in situ signatures at Earth. We find a mean arrival time
difference of 6.5 hrs between predictions from the two different viewpoints,
which can reach up to 9.5 hrs for individual CMEs, while the mean arrival speed
difference is 63 km s$^{-1}$. An ambient solar wind with a large speed variance
leads to larger differences in the STEREO-A and STEREO-B CME arrival time
predictions ($cc~=~0.92$). Additionally, we compare the predicted arrivals,
from both spacecraft, to the actual in situ arrivals at Earth and find a mean
absolute error of 7.5 $\pm$ 9.5 hrs for the arrival time and 87 $\pm$ 111 km
s$^{-1}$ for the arrival speed. There is no tendency for one spacecraft to
provide more accurate arrival predictions than the other.
