Ok, I'm going to go straight to the RAW for this. YS278
The one restriction on the bonuses provided
is that they may not total to a number greater
than your Lore. So if your Lore is Good (+3),
you can have an evocation focus item that
provides +3 to offensive control, offensive power,
defensive power, or defensive control, or a focus
item that provides +1 to three of those, or +2 to
one and +1 to another, but you can’t construct
one that provides bonuses totaling 4 or more.
So the lore limit applies to the total bonus that a single focus item can have. In addition you can have both control and power bonuses in a single item, however that lore limit still applies to the single item. So the advantage to using two items would be that you could have two higher bonuses (a +4 and a +4) in two different items rather than two lower bonuses (a +2 and a +2) in a single item. I can't find the specific rules that say that you can have both control and power in a single item, they're probably on that same page, however there is an example which demonstrates it.
So an item
that offered +1 offensive power and +1 offensive
control to fire and earth evocations would
take up 4 slots.
So I would assume that it's possible to have bonuses to power and control in one item.