How do you get Palkia and Dialga on platinum?

Answer 1

You have to beat the elite four and cynthia, then go to the waterfall in mount coronet then go up the waterfall and into the cave. Pick up the two items (The Adamant and Lustrous orbs) then go to the spear pillar and talk to the blue light and you battle dialga once you catch/defeat dialga go back into mount coronet and go back to the spear pillar and talk to the pink light and you will battle palkia (remember to save before the battles)

Answer 2

Once you have the Adament and Lustrous Orbs, go to Celestic Town and Talk to the Elder. Go to Spear Pillar and talk to a Blue Light and Dialga will appear. Go back into Mount Cornet and come back and a Pink Portal will be there. Talk to it and Palkia will appear. They appear at level 70, so come prepared. Please note that you must have beat the Elite four and the Champion. Hope this helped!

Answer 3

all you have to do is go to spear pillar after you pay a visit to girratina in the reverse world and go into the portal then go back in mt. coronet come back out to spear pillar and then go through the other portal

p.s. the first portal will be dialga the second is obviously palkia hope i helped