You most certainly could do that. If you choose to do so and want them back as one work layer, you can do that also. If you have one of your harvest layers visible, click on the Edit button below the lower/right corner of the map, then Merge Work Records. On the screen that pops up, select the work layer you want to merge together and click Merge Work Records in the lower right corner.

Another way to do what you're referring to is to bring up your harvest layer on the Analyze map window. Display the Harvest Date (you might have to click on Show More to see the Date layer). Select an area in one of the specific day areas on the map with the Selection tools on the right side of the map. The Selected zone icon right above the flag is what you use to select areas, there are different options that pop out when you click on it. The summary section below the map will now show info for the area you selected. repeat the process with another area that you harvested on a different day. This might be a better way since you aren't having to break up the layers and merge them back together.

Just a couple of ways to analyze like you talked about doing

It would be nice to have the ability to use the compare function in the Analyze window to compare by different harvest dates, but that is not an option that I can see. The Compare screen is for comparing different layers in the same area. If, for example you did a planting prescription at different populations (I did one in the middle of a couple fields to see what difference soybean population had on yield). You can draw around one area, see what the yield was there, and draw around another area right nearby in a different plant population and see what it yielded there