one=source1
two=source2
three=source3
four=4

two_+_four=%(two)s + %(four)s

[X]
x.one=conf2-sourcex1
x.two=override-conf2-sourcex2
x.three=conf2-sourcex3

x.four=exists here only

[Y]
y.one=conf2-sourcey1
y.two=conf2-sourcey2
y.three=override-conf2-sourcey3