That is a bit odd to do just one at a time. .
You can see with some breast exams on youtube,as well as in the movie The Hand That Rocks The Cradle, they do one at a time when the patient is lying down, to minimize on the exposure.
The visual part to look for symmetry & all that stuff is usually done sitting up, & i see both are always exposed while sitting usually. But when palpating etc, since only one is being worked on at a time, they usually try to keep the other one covered.
I found it a little ridiculous how my doc the other day made a point to cover me after applying each EKG electrode to my side, especially since it was quite apparent that i was not the least bit concerned about exposure. In fact, being covered by the gown worked against her, as i perspired, thus making it hard for the electrodes to stick.
Didn't mean to tangent off the topic, the point being the lengths health care professionals go to to maximize the modesty factor, even when patients like me have zero sense of modesty. Thus, while lying down for the breast exam, you'll notice many docs will keep one covered while examining the other.
Yes, i used "tangent" as a verb. Poetic license. 😉