We've been selling that product twice as fast as we usually do today.
That product is selling twice as fast as usual today.
この場合、次のように言えますよ。
ーWe've been selling that product twice as fast as we usually do today.
「今日はいつもの２倍の速さでその商品を売っています」
もし話してが店員で自分の店で売っている商品のことを言っているのなら、このような言い方ができます。
to sell ... twice as fast as we usually do「…をいつもの２倍の速さで売る」
ーThat product is selling twice as fast as usual today.
「今日その商品はいつもの２倍の速さで売れている」
... is selling twice as fast as usual で「…はいつもの２倍の速さで売れている」
ご参考まで！
カナダ 
