You've miscalculated in the first instance.
$400k+$100k-$412k (threshold) = $88k x 1.6% = $1408 payable by A
Case b) $400+$99 = $499k x 1.6% = $1392 payable by A
A saving of $16
If however they purchase both in 50/50 shares then each would pay $38k x 1.6% = $608 each or $1216 total.