You'd be better off, and likely have less headache to utilize
the Domain Access module (
http://drupal.org/project/domain)
to
achieve the latter bit, rather than separate installs, sharing a
database. I've built a multinational, multilingual storefront with
Drupal and Ubercart earlier this year and that was definitely the way
to go - you can manage content per domain, or make it available for all
domains, use different themes per domain, etc. If nothing else, come
upgrade time you will thank yourself for doing it this way as you'll
only have one site to maintain and upgrade - plus then your admin
section IS in one spot, rather than having to fake it together.
As for general store/reporting separation, take a look at
Ubercart Domain Access (
http://drupal.org/project/uc_domain)
which
claims to allow "...an Ubercart store to span multiple domains,
storing the originating domain when an order is made and displaying the
correct store information on invoices."
I'm sure you'll have to do a little bit of exploration to make
it all work together happily, but venture to guess it will be less time
than the other approach.