turing machines – Show that a language is not decidable by reducing from ATM

Let (ATM denotes the language ${langle M,w rangle mid text{TM $M$ accepts $w$}}$)
show that the language L={<M1,M2,w> | M1 and M2 both accept or reject w} is undecidable by reducing
ATM to L
Not really sure how to approach this. Any hints or pointers