DropDownList, select two values to insert a third value error

i am trying to select a fixture date in combination with the team playing on that date in a dropdownlist, which when selected will then insert the fixture ID associated with the details in the DDL to an sql table on button click.

however with the method i try i get the error 'The data types date and varchar are incompatible in the add operator.'

<asp:DropDownList ID="DropDownList2" runat="server" 
                    DataSourceID="SqlDataSource2" DataTextField="Fixture_ID" 
                    DataValueField="FixtureID">
                </asp:DropDownList>
                <asp:SqlDataSource ID="SqlDataSource2" runat="server" 
                    ConnectionString="<%$ ConnectionStrings:loughtonccConnectionString %>" 
                    SelectCommand="SELECT ([Fixture_Date] +' '+ [Team_Name]) as FixtureID, [Fixture_ID] FROM [Fixture]">
                </asp:SqlDataSource>

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