yes, they can use either side. Since its 3/8 if robbing may be an issue you can use and entrance reducer. I keep most of my hives at least 30 feet apart to minimize robbing. I usually put it between the supers and the deeps. the additional "space" acts as a queen excluder. I don't put on more than 2 supers usually, but if I do I will add another entrance above. The goal is to have basicly 2 seperate work forces. honey gathers, and brood tenders....... and to make each groups job uncluttered..... the hives I do this on seem to produce more brood as well as more honey, but that may be from amny other things and I would need to set up a study to prove it.... (not worth it to me) So may say that its too much open space, but after looking at several dozens of wild hives I say baloney..... most wild hives have 6-8 inches of open space.... some are almost totaly open.
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