Surface finishes are notoriously difficult to get smooth when the process is interrupted. The glossier the finish, the harder to have sections blend.
If you are talking about preliminary coats, especially if they will get a light sanding between coats, this is probably ok. For the finish coat, I would strive hard to do it all in one shot.
If you simply cannot, try to find a transition area that will be less noticable, such as an area covered by rugs or furniture. You also might try cutting the transition from one section/session to another along a long board line, but this is hard with poly.
@iLikeDirt covered a lot of important information, but I'd like to add something since the majority of the information assumes that there is a problem of excess moisture due to the concrete, while the expert assessment says it is actually dry cupping that is the problem.
Dry cupping implies that the interior humidity is dropping below the average frequently enough that the wood surface dries out. It sounds like they are suggesting that the vapor barrier between the slab and floor is probably doing its job correctly.
One way to help prevent this is to bring the wood into house and allow it to be exposed to the air within the home so the moisture content is similar to that within the home (acclimation). I like to assume this is standard practice and this was already done, but if you're using a special reclaimed antique pine wood, then maybe a longer period would have been desirable. Major seasonal humidity variation can still have an impact.
Do you know if there is a period during the year where it seems more prominent, or has it not been long enough? If I had to guess, it could be during the summer when the AC/dehumidifier is running more frequently. I'm not as familiar with the climate there, but in parts with cold winters you are more likely to encounter dry air issues during the cold months.
One thing that could be happening is that the glue on the underside of the wood is acting as a seal on just that side, inhibiting the wood from 'breathing' underneath. The only surface area for moisture to leave the wood is on top. A solution that allows the wood to better breath on the bottom, or simply avoids using glue or too much glue, might help. But then you need make sure the vapor barrier is doing its job or you will end up back where you started with too much moisture underneath, the expert seems to imply you no longer have that issue though.
You can still end up with this problem with engineered floors. It could still be a problem with an unknown source of moisture throwing things out of balance as well. I'd see what the neighbors are doing as a solution for your area and if they have similar problems before I spend more money.
Best Answer
Yes, but you better be right on the money with your re-coat times.
A second layer of paint bonds to the first layer one of two ways. Either Mechanically by the paint grabbing onto the last coat, or chemically by the paint diffusing into the last coat.
"Chemically" requires that you hit very specific "re-coat windows" that are documented on the paint label/instructions.
"Mechanical" doesn't work on gloss paint, because it has a finish like glass.
Their use of flat paint is partly to "cover" them against missing their re-coat window.
However gloss paint can be made re-paint-able by scuff-sanding it, i.e. sanding it with something like a Scotchbrite pad to remove the gloss. (if you're removing layers of paint at corners and edges, you're sanding much too hard). This roughens the surface to give it some "tooth" for the next coat to bond to. I am the master of missing my re-coat window, and that's how I handle that.
Paint also tends to be quite soft and vulnerable during the re-coat window, so unless you're Tom Cruise, painting a floor might be a bit of a problem!