Leyline of the Void and Serum Powder

[vinnymack]

I assume when you start with both in your opening hand you have to choose 1 or the other, but likely don't have the option to do both

[Forbiddian]

I assume when you start with both in your opening hand you have to choose 1 or the other, but likely don't have the option to do both

Leyline only works after both players have finished mulliganning.

Serum Powder obviously has to take effect during the mulligans.

[cdr]

103.5. Once all players have kept their opening hands, if any cards in the starting player's hand allow that player to begin the game with those cards on the battlefield, he or she may put any or all of them onto the battlefield. Then each other player in turn order may do the same.