From the numerical point of view пара by itself may mean:
- one object: пара ножниц, пара щипцов, пара брюк;
- two objects: пара яблок, пара апельсинов;
- one set of objects: пара весел, пара перчаток, пара носок, пара платья (outdated nowadays, used to mean "сюртук, брюки и жилет", see here), кофейная пара (чашка и блюдце для кофе), пара чаю (порция чаю в двух чайниках для воды и для заварки чая);
- two humans or animals acting together: пара лошадей, пара танцоров, пара боксеров;
- small amount: на пару слов, на пару минут, пожить пару дней, виделись пару раз, подкинь пару монет.
According to a joke the "small amount" may mean even 5 objects:
– Сбегай в магазин, возьми пару бутылок водки.
– "Пару" – это сколько?
– "Пару" – это пять!
Now, back to две пары. Though mathematically it can be treated as a simple multiplication, though in language such saying has multiple additional aspects.
Let's use десять or десяток instead of два to highlight the difference. While Then десять пар means:
Пара as one object. In our first example math works. In this case десять пар means exactly ten times more objects than before (ten objects): десять пар ножниц, десять пар щипцов, десять пар брюк.
Пара as two objects. In this case math works as well, but semantically it is implied that the objects are calculated by pairs for some reason (because of packaging, or because price is set for pair): десять пар яблок equals 20 apples, десять пар апельсинов equals 20 oranges.
Пара as set of objects. Here math works differently:
Она перемеряла десять пар туфель - you cannot just multiply two by ten here. Though 20 shoes were used 10 of them are left ones and another 10 of them are right ones. You cannot take one shoe from first pair and combine it with another shoe from another pair easily.Десять пар перчаток - same as above - 20 objects, but left/right glove are different.У меня десять пар одинаковых черных носок - Maybe the only example when you can say that you have 20 socks, because usually there is no practical difference between 'right' and 'left' ones.В гардеробе висят десять пар платья - in this example mathematically you have 30 objects, but they cannot be treated as equal.
Пара as two humans or animals acting together - though you can abstractly say "20 humans" or "20 animals", but in fact it is another example of "пара as set". When something is done together, in most cases these pairs are matching each other by some criterion (horses should be of equal sizes and strength, boxers should be in same weight category; dancing pair may or may not require one man and one woman, etc.). So in most cases you cannot mix the pair members.
Пара as small amount - you cannot use пара as small amount meaning in десять пар минут expression. Here it will mean пара as grouping by two for counting, so десять пар минут means 20 minutes.
Back to две пары часов. Here the word пара can mean one, two or set. The word часы may mean clock or wrist/pocket watch or hours.
In case of hours две пары часов sounds quite weird by itself, because usually пара часов is used, which means a couple of hours, but две пары часов will have the meaning four hours, grouped by two hours for some reason (note that in universities пара stands for two academic hours - a study session in an educational institution consisting of two 45-minute periods).
In case of clock пара means exactly two clocks, so the meaning here is:
Four clocks, that were grouped by two either for ease of counting, or because they form a set by their form, color, price, etc.
Though since for watches the result is quite different one would likely add "настенных" to disambiguate clocks and watches (две пары настенных часов).
In case of wrist/pocket watch пара may mean:
Two watches (wrist or pocket ones) that are weared by a person. This is a colloquialism - most books on Literature Russian discourage usage пара in this meaning. As stated by Rosenthal: "Выражение «пара часов» имеет просторечный характер". However, this expression follows a pattern for many other clothing items - пара очков, пара брюк, пара носок, пара перчаток, etc., so it does not sound weird for a person that have never heard such expression before. Anton Zuev listed a numerous examples of such sayings. One example from ruscorpora.ru dates back to 1874:
Пришедшие под вечерок армяне «поддержали коммерцию», купив пару часов и несколько мелочей из сданных мною вещей, как то: цепочки нового золота, коробку слабителъных порошков, перочинные ножики, карандаши и т. под. [П. И. Огородников. Очерки Персии. Калейдоскоп шахруда. Персидское побережье Каспия. Перевал через Кузлук (1874)]
It is interesting to note that in this citation the pocket watches are described. According to Wikipedia wrist watches become popular after World War I, because pocket watches were impractical on the battlefield. Before that time men, who carried pocket watches, were quoted as saying they would "sooner wear a skirt as wear a wristwatch" (note this idea of "watch as a piece of cloth" connotation, though the citation itself is not russian of course).
Four watches, grouped by two for counting (see "clocks" example). I beleive that this is the only meaning that Literature Russian allowes. So, such phrase can only be used in a context where these watches are counted (in a shop, in a document, etc.). But since alternative meaning exists that has a very long usage history, I believe that you have a very low chanses of seing such expression in literature, because most editors will rephrase it to avoid ambiguity.
So, as a bottom line, две пары часов, where часы means watches (and not clocks or hours) will always mean two watches.
