Yahoo pipes - any way to combine items?

List 1:
[0] letter - a
[1] letter - b
[2] letter - c

List 2:
[1] word - apple
[2] word - banana
[3] word - cake

List 3:
[1] letter - a
    word - apple
[2] letter - b
    word - banana
[3] letter - c
    word - cake