In the epsiode "Bart Gets Hit By A Car".When everyone is gasping because of Bart's Tale of his injuries you can quickly see both of the twins from "Life in Hell"(a comic that Groening pulished).They are fairly far back but you can reconise them because of their hats.You can also only see them for about a second or two because the "Gasp!" part is pretty short.Good luck and keep your eyes peeled.

P.S:You may want to put the entire "Gasp!" part in slow-mo so you can see them.