Kevin, I have hesitated to reply regarding mites, it is a complicated subject.  Mite levels in a typical hive increase from summer to fall.  But, you can have high enough mite levels in the spring to cause serious damage to your hive. High mite levels on adult bees expressed as a percentage would be around 5% or greater.   In other words 5 adult bees in every 100 has a mite or mites on it.  (depending on the time of year, even mite levels below 5% can be bad and need to be treated)  This doesn't seem like much.   The catch is that at any one time in a typical hive 2/3 to 4/5 of the mites in the hive are in the brood cells feeding on the larvae and reproducing (multiplying) .  They don't get counted as a part of that 5%.  But they are there doing damage to the brood.  So, treatment is based on mites you can count fairly accurately and extrapolated (interpreted?) to allow for mites you can"t count.

Mites harm the bees by weakening the adult bee or bee larva's immune system, and introducing viruses into their blood stream. The resulting symptoms can be brood disease, poor queen performance, low hive population, slow buildup of population and other conditions interfering with the overall health of the hive.

Methods have been developed to take a sample of bees, usually around 300 bees, from a hive and separate out the mites for counting. I won't go into the method here, but it is low tech and fairly easy to do with the right equipment.  You can read about it on line, or maybe experience it sometime this summer with a local bee keeper.  If the opportunity arises to attend a "Saturday in the Apiary" with Heike in Madras, do it if you can. ( This program is provided by the OSU Extension Service) Heike usually demonstrates mite counting several times during the summer season. That opportunity may not be available this year considering the COVID 19 situation. 

Treatment recommendations for mites are based on your hive's mite count.  There are around 5-10 methods of treatment, mostly what I would call chemical, which each has it's own advantages and disadvantages. Some are more effective than others depending on when or how or how much is used of a particular treatment and the overall effectiveness of the treatment itself.  Some require safety equipment, some can be applied when honey supers are on the hive, some are only permitted to be used when honey supers are not on the hive.  It is really necessary for you to read up on the subject in order to make choices for yourself and your hives.  After you have a grasp of what some of the options are, then you can have a conversation with beekeepers who have some experience with the different procedures. I again refer you to the honeybeehealthcoalition document in my earlier post as a starting point.  It covers the mite counting process, and the pluses and minuses of different treatment methods very well.

In general, I usually count mites 3 times each year. Spring(April or early May) mid-summer(June or early July) and late summer(August or early September). (Of course I am retired and have lots of spare time ;-) Then I immediately treat the hives that need treating.  I mainly use formic acid in the form of Miteaway Quick Strips or Formic Pro to treat.  I occasionally use other methods also.  

Treatment once a year in the fall isn't usually adequate to keep the mite levels low enough to keep your hives healthy. 

I hope I have informed you more than I have confused and discouraged you.  Good luck with your hives, Dennis 

Kevin, I agree with Allen on letting the hive fill the boxes to 70-80% before adding another. Also if your hive hasn't filled the two deep boxes by mid September, feed them sugar syrup (2 sugar to 1 water mix) until the boxes are full or they are no longer taking the syrup and storing it because it is too cold. Depending on when you last kept bees you may or may not be experienced at monitoring mite levels.  In central Oregon (and most other places), it is just as important to the survival of the hive through the winter to keep mite levels under control. If you need a refresher on mite monitoring, look to https://honeybeehealthcoalition.org/wp-content/uploads/2015/08/HBHC-Guide_Varroa-Interactive-PDF.pdf  This is a very good reference document for mite monitoring and treatment.  COBKA will also undoubtedly be talking about mite monitoring during the summer.  Stay tuned.

Kevin,

People here have been successful with both although in my opinion you have to work a little more with the single hive body method.  I personally overwinter with two hive bodies (preferably completely full of honey in the Fall).  I find that usually gives them enough food to make it through March when the natural food becomes available.  I find this gets them going earlier in the Spring.  When doing a single box, you'll want to have a race of bees that aren't particularly quick with the spring build up (starting raising brood in Feb and starving), and that have a smaller winter cluster.  You probably should check stores monthly during the winter as well.