I just had a Yanmar surveyed, but the shop that did it also a certified on Cummins. They charge around $1,600 for two engines and a generator. I paid a little over $850 for a single Yanmar and a generator with taxes in New England. This included oil samples.
I would schedule the Hull Survey first. Then if the hull passes, schedule the engine survey during the seatrial. I brought the hull surveyor back for the seatrial despite having an engine surveyor onsite. This was mostly because the surveyor couldn't asses the systems that require the boat be in the water. (head, raw water washdown, depth sounder, auto-pilot, Heat and A/C, etc).
This is fairly common. These local guys all know each other.