Do margin debit balances and short stock collateral cancel each other out?

$2M long stock
-$1M margin
$1M nav

If I then short $1M worth of stock is my effective margin $0?

 

No you need to provide more margin.

 

Long Stock + Long Options - Short options are part of the Type 2 Margin account. Short stock is a Type 3 account and does not provide an offset to prevent borrowing funds. If your Type 2 balance exceeds your equity, you are borrowing money.