If you are seeing Poly as missing two nights in a week now in Sep, that indicates you are looking at late Sep for when you plan to go. That is the beginning of DVC's very high demand period that lasts until early Jan. Thus, if you intend to reserve any part of that time, particularly for Poly and including lake view, be advised you may need do it quickly because more days could soon disappear. Moreover, if your co-worker does not own at both of those resorts, he will be limited to reserving at 7 months out, by which time even Poly lake view could be gone for the nights you want and even Kidani has some risk of a night or possibly two disappearing. In other words, you may need a back-up plan.
As to splitting the stay as you propose:
1. The studios at Poly are huge (close to 450 sq ft) in comparison to Kidani (near 360). Thus, if you start at Poly and move to Kidani, you could have a depressing room-size shock, so you may want to do Kidani first (unless you are intending a 1BR at Kidani, which are over 800 sq ft, have a full kitchen w/d, and two bathrooms, and thus the room size and amenities shock could be the opposite, indicating Poly first).
2. You mention Poly lake view. Be aware that those are on two floors at a building with the ground floor view not being as good as the second floor view (some have phrased ground floor as a real disappointment as lake view), and though your co-worker can put in a request for second floor, there is no guarantee you will get it.
3. Both are excellent resorts but very different. Poly is in the midst of the "action" around MK and gives you easy access to Epcot, MK and the other MK resorts (and thus numerous restaurants) via monorail, the ability to watch fireworks from your resort, take in the Polynesian luau, and it has a huge lobby and very good and large stores. Kidani is themed as an African savanna where you get to watch animals, which theme includes creating the sense that you are actually on an African savanna far removed from the rest of the world, has one very good table-service restaurant, a fairly small lobby, and very small store, and only bus service to all the resorts. You also have access to Jambo and its large store, large lobby, and three restaurants, including Boma one of the best buffet restaurants on site, but the distance between the buildings, front door to front door is about a half mile, potentially making the walk long, although a bus runs throughout the day and evening between the two buildings. The pools at both are good with Kidani having an excellent water-play area for kids.
4. Some like split stays, some do not (I am in the latter). The hassle of switching is not overwhelming (the possible overwhelming part is that you need to be with those who can pack quickly -- and I have never had that good fortune with any in my group) as Disney moves your stuff from one resort to the other, but you may not be able to get into the room at the second resort before 4 p.m., so you should likely plan to be at a park for the day when making the switch.