coding mixed contrasts


i have problems coding the syntax for an anova with two factors, one between- and one within-subjects-factor (see syntax below). accordingly, there are 4 combinations of levels:
wth_level1-btw_level1, wth_level2-btw_level1, wth_level1-btw_level2 and wth_level2-btw_level2.
i am interested in the contrast of the two middle combinations (wth_level2-btw_level1, wth_level1-btw_level2) vs. the first one (wth_level1-btw_level1).
could anybody help me writing the syntax command for this contrast? this would be great!
thanks in advance! best, danihu

GLM wth_stufe1 wth_stufe2 BY btw
/WSFACTOR=wth_faktor 2 Polynomial
/WSDESIGN= wth_faktor