Their formula will be #XY#.
Notice that the number of electrons located in their outermost shell varies significantly for the two elements.
On one hand, #X# has only 2 electrons in its outermost shell. This means that it's easier for #X# to lose these electrons In order to complete its octet, i.e. have a total of 8 electrons in its outermost shell, than to gain 6 more.
On the other hand, #Y# has 6 electrons in its outermost shell, which means that it can easily gain two electrons to complete its octet.
Element #X# can thus form 2+ cations, #X^(color(red)(2+))#, and element #Y# can form 2- anions, #Y^(color(blue)(2-))#.
When these two elements react, #X# will lose its 2 valence electrons and #Y# will take them. The two elements will thus form an ionic compound with the formula
#{: (X^(color(red)(2+))), (Y^(color(blue)(2-))) :}} =>X_(color(blue)(2))Y_(color(red)(2)) <=> XY#
