I think that this may be achieved by editing the httpd.conf file of Helicon ISAPI Rewrite 3, the 3rd party software that uStore and uProduce use to rewrite incoming URLs.
The appropriate instructions added to this file would redirect the visitors of the original Store URL to a home-made HTML page, where 2 buttons or links would in turn let the user visit their preferred Store.
May I highly advise to consult with the XMPie Support Team or somebody who is familiar with such files and regular expression as well, as they are sometimes way to be readable?
Also, please notice that this file will be modified, as soon as you change a Friendly URL in uStore Admin. So, if any, make your manual changes only before the "####USTORE_BEGIN####" tag or after the "####USTORE_ustoreMap.NoTenant_END####" tag; if you don't, there is a risk that your editions would be unexpectedly overwritten by uStore Admin.
Good luck!