Build a php/html file to have drop down from 2 MYsql tables

Databases
1

hi,
I am trying to have a input php/html file that contains drop down list from 2 different tables.

Table Name | Column1 | column2
customer | Cust_id | customer
Products | Prod_id |Item

I need to be able to select records from these table to insert in a third table (Orders)

here is my querry but I am not getting the right data.

SELECT Company from customer
UNION
SELECT Item from Products

also I tried :
SELECT COUNT(*) AS Rows, Item, Company, Type FROM test3 GROUP BY Item ORDER BY Item

can someone help please?