  We present sub-milliarcsecond observations of SiO masers in the late-type
stars IRC +10011 and Chi Cyg. We have used the NRAO Very Long Baseline Array
(VLBA) to map the 43 GHz (v=1, 2 J=1-0) and the 86 GHz (v=1, 2 J=2-1) SiO
masers. All the transitions have been imaged except the v=2 J=2-1 in IRC
+10011. We report the first VLBI map of the v=1 J=2-1 28SiO maser in IRC +10011
as well as the first VLBA images of SiO masers in an S-type Mira variable, Chi
Cyg. In this paper we have focused on the study of the relative spatial
distribution of the different observed lines. We have found that in some cases
the observational results are not reproduced by the current theoretical pumping
models, either radiative or collisional. In particular, for IRC +10011, the v=1
J=1-0 and J=2-1 28SiO lines have different spatial distributions and emitting
region sizes, the J=2-1 emission being located in an outer region of the
envelope. For Chi Cyg, the distributions also differ, but the sizes of the
masing regions are comparable. We suggest that the line overlaps between
ro-vibrational transitions of two abundant molecular species, H2O and 28SiO, is
a possible explanation for the discrepancies found between the observations and
the theoretical predictions. We have introduced this overlapping process in the
calculations of the excitation of the SiO molecule. We conclude that the line
overlaps can strongly affect the excitation of SiO and may reproduce the
unexpected observational results for the two sources studied.
