You can connect multiple Xero Organisations to a single UpSheets account.
To do so, simply select Organisations from the menu and click the Add Organisation button (see here for detailed steps)
You may also assign users to each organisation to give them access.
While there are no charges for adding additional organisations or users to your account, the usage for both will be added to your subscription for invoicing.
If you require separate invoices/accounts for each of your organisations, then you are required to create a different upsheets account and subscription for each one.