Can boots of speed be activated twice in a round?
Say for instance you are under the effects of slow. So you activate the boots to counter/dispel slow. Are you now able to activate them a second time (using a second charge) to now be under the effects of haste for the one round?
Boots of speed are one of the few items which are activated as a free action which why this could be possible.
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.
(emphasis mine)
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.
If the GM rules that taking a free action to activate the boots of speed dispels a slow effect affecting the wearer—in the same way that a haste spell automatically dispels a slow spell—, then the boots' wearer may be able to take a second free action to activate again the boots of speed so that the wearer can also benefit from the haste effect the boots provide… so long as the GM hasn't put an arbitrary limit on the number of free actions a creature can take on its turn, a right the GM can exercise.
Whether activating the boots twice in a round—once to dispel the slow effect and a second time to get the haste benefit—spends 1 or 2 rounds of the boots' 10 rounds per day is for the GM to decide. (This GM would rule the wearer spends 1 of the boots' rounds to dispel the slow effect then spends 1 of the boots' rounds to benefit for 1 round from the haste effect, but this reader can easily imagine an argument for both effects combined spending only 1 round of the boots' 10 rounds per day.)
To be clear, if the GM rules that the effects of the boots dispel a slow effect, that slow effect ends instantaneously: the slow effect isn't, for instance, merely suppressed while the boots are in operation, but, instead, the slow effect affecting the wearer is just straight-up gone.
Note: More about how spells counter and dispel other spells here. More about boots of speed is here.
Haste and Slow spells are listed as counters of each other yes, but both also mention that multiple castings of the same spell don't stack. As the boots provide a haste effect, but spread it over the course of 10 rounds of the wearer's choosing in a day it is not cleanly comparable to a single casting of the Haste spell which lasts 1 round per level as does the Slow spell.
The most logical course of action is to have the activation of the boots temporarily suppress the Slow effect. This should be on the boots' wearer only (not other targets, and definitely NOT dispelling the Slow spell), and for only those rounds that the wearer chooses to be under the effects of the boots.
If the boots' wearer were to attempt to click them (activate) again while they were already temporarily suppressing the effects of the Slow spell then this would be an attempt to stack the Haste effect which is specifically called out as not being possible in the spell's text.
