From the description of Boots of Speed,
As a free action, the wearer of boots of speed can click her heels together, letting her act as though affected by a haste spell for up to 10 rounds each day.
Note that you do not expend a charge to cast a spell from the boots, you activate them instead which lets you to act as if affected by a spell for a round.
Basically, the boots have two states: inactive and active, and they can remain in active state for up to 10 rounds per day. There is no "double active" state that expends two charges per round anywhere in description.
I would also note that since Boots of Speed don't actually cast a spell but activate a spell-like effect on wearer, the interaction between them is more complicated. I couldn't find any FAQ or RAW on that, but I personally would rule that the slow effect is not dispelled but suppressed while the boots are activated. YMMV.