I bought an AeroSkull HD from Jarre Technologies that already had this feature and I remember that they called it TrueWireless Stereo. It's a proprietary technology developed by CSR, the company that makes most of the Bluetooth chips and who have previously acquired aptX.
Unfortunately this particular feature is not advertised through ingredient branding, contrary to the original plans for aptX. Finding more information about TrueWireless Stereo seems to be rather difficult. CSR has a separate website for aptX which I suspect to be closely related, but there is no information either, only categories for different products without filtering and sorting to display newest products first or to only show products with that particular feature. (Most of the products on the first page look old.)
Price comparison sites like geizhals.at also have no information about this feature, though this site usually is very well organized – you just select features you want in a product and get the results.
The best I can find is the following video from CSR:
I saw that some similar Stereo functionality was also advertised for newer portable speakers like the Sony SRS-X11. While the AreoSkull and some older speakers already have this feature I'd recommend to look closer at products released in 2015 and later.