As I roll up I'll press my OnStar emergency button, if they answer before I stop I'll tell them officer down, send the cavalry, otherwise it will be an open line and they should dispatch help when there is no answer.
I'm likely not to go hand to hand, this person just overpowered a uniformed officer and trained or not I'm not all that big and I have other tools at my disposal. My gut says to draw with my right hand, OC in my left hand and order him off the officer and down on the ground. (I may not be big but you don't want to be on the business end of my mean, loud voice). If he doesn't comply he'll get a full dose of OC. It may affect the officer but he's likely out of the fight already. I'm just hoping to get the guy to stop long enough for the officer to hopefully retreat. If he still doesn't comply he'll get one more command "stop or I'll shoot" and then I'll let my Sig do any further talking, all from a safe angle to not risk injuring the officer. Once the threat is stopped I'll get of the officers radio if it's safe to do so and call in officer down, shots fired, assuming he hasn't already done so. Then I'll tend to any injuries the officer has until the paramedics arrive.