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).