A single query to get data from two tables would look something like this:
"SELECT * FROM customer INNER JOIN pet ON (customer.customerID = pets.customerID)"
In the example above I am assuming your customer table is called "customer" and your pet table is called "pet". I am also assuming that the column that links the two tables together is the customerID. For instance, each pet entry has a customerID associated with it to link it to a customer.
For your second issue, your record set for the various customer back end pages needs to filter by the customerID.
For instance it may look something like:
"SELECT * FROM customer WHERE customerID = '1'"
Of course there are many ways to get the customerID dynamically via either a POST from a form or GET from a form or a session variable etc. I believe by default dataassist uses GET. So in Dreamweaver for your recordset you would use a setting of: "Filter by customerID using URL = customerID"