For something like that, I'd go with a spirit maneuver, plain and simple. There isn't really a rule for allowing someone multiple actions during one exchange, and I think that's a good thing. With a maneuver spell, your ally can tag the resulting aspect on his attack and you can flavor it as time speeding up around him or slowing down around his target.
Any other way to implement such an effect with the methods available would probably be a block. You could set up a block around your ally. If it is strong enough, nobody will be able to hit him, which would be the same effect as if they didn't do anything. Unless they maneuver, of course, but then you could (at a much higher power) put up a block on them, which can get into pretty large numbers, depending on the number of opponents.
And then there's the old "tag for effect". Do a maneuver spell as above, but instead of tagging it for a +2, you tag it for effect. The effects will vary, depending on what you agree upon with your GM. That may or may not be a second action in one exchange. A good compromise would probably be to even things out, timewise. The tagging character would get his action from the following exchange right now, but that would mean he loses his next action. Time speeding up for him, and then swinging all the way back, slowing it down, and then working as usual again.
