Ah, this is fun. Let's see what happens when we google 'band member wearing other bands'.

wearing Motley Crue

wearing Joy Division

wearing One Direction (with Zayn crossed out)

wearing KISS

Same thing

in Iron Maiden

same again

wearing Notorious B.I.G.

wears Slayer

in Blink-182

in Guns'n'Roses

takes it to another level by being on the cover of Rolling Stones magazine wearing Cramps

wearing Taylor Swift

wearing The 1975

If you know any other examples of this wearing-each-others-merch: please, please, pretty please, send those our way (like on Twitter maybe).