In skin1/customer/main/product.tpl
Change...
Code:
{section name=quantity loop=$mq start=$start_quantity}
To...
Code:
{if $cat eq "123"}
{section name=quantity loop=$mq start=$start_quantity step=2 start=2}
{else}
{section name=quantity loop=$mq start=$start_quantity}
{/if}
"123" being the category ID.