I'm running the Global Cart option for a subdomain multisite install - it's exactly what I'm looking for with products unique to their own site, but funneled into a single shopping cart.
However, it looks like the only payment gateway in this configuration is PayPal Express. Is there a way to select another gateway, like Authorize.net AIM?
I'm not concerned about the idea of grouped payments that PayPal offers. In this case a single payment is all that is needed because the products across the different sites are all being sold by an individual company.