If you are trying to figure out how to combine data from multiple tables into a single datagrid, there are two ways to do it: change your query string or manipulate the data within C#.
If you are trying to show one column with all the names from the two tables combined: (a) The SQL way is to use a union to combine the data from two tables, such as "select naam from items union select naam from combo" (b) The .Net way is to take the results from two separate executes and append one to the other like
DataTable dt1 = GetDataTableFromQuery("select ... from Items");
DataTable dt2 = GetDataTableFromQuery("select ... from Combo");
(note, the above is oversimplified, but gives you the idea.)
If you want to show data from both tables in the same datagrid, such as column 1 is Items name and column 2 is the Combos Name, then (a) SQL way: join the two tables using the Combineren table like "select Combo.Naam, Items.Naam from Items Inner join combineren ... inner join Combo ..." (b) The C# way is just ugly. Trust me, just try to use joins. I'm not sure on Join syntax for Access but that's really what you want to use.