This depends on how the panels are constructed to begin with. However, if anything, having two ballistic panels is probably slightly less effective than having a single panel that is twice as thick.
Keep in mind that Kevlar panels are made of several layers of fabric to begin with. Frequently the layers are only tacked to each other to make the vest flexible and thus more comfortable to wear.
Unfortunately if the layers separate, the vest is more easily penetrated. This is well known and has been the subject of much heated argument between the NIJ and various body armor manufacturers (in reference to testing procedures).
The ballistic effectiveness of the panel can be improved by cross-stitching the layers to each other. However, this makes the vest stiff and less comfortable.
Thus it seems likely that the looseness between the two separate panels would make it slightly weaker than a single panel of twice the thickness.
Further, as e8ght suggests, wearing two separate vests is likely to be totally unbearable.