Oh we are definitely giving this ability to both players. Would add a nice strategic element - at what point to use your aid. And while players are fighting, they might wanna send Alliance message asking their team members to help them.
But it will be limited - to 10 units tops, and no OP stuff, we`ll be sure to balance it right. Basically as most it would make 20% difference on the battle - depending on how big the players original fighting force is.
And good idea - we could make this ability recharge in 10 minutes.