Using actual <fieldset> elements would require a substantial amount of customized PHP coding to activate exactly at the right places.
Mimicking that effect is doable with stylesheet rules for the particular items in Flexible Attributes. I could give specific examples if I can see your Zen Cart site live. Essentially, you would give the left item in a group a comment in admin for the "fieldset" name, then position it with styling and give the item borders on top, bottom and left. The second item would get borders on top, right and bottom. Both would have to be given explicit heights and probably widths to ensure that their borders align. There may be a more elegant way of doing it using padding on the left item and absolute positioning on the right item.