The ControlToCheck property allows the spellchecker to spellcheck only one element on time.
If you want to spellcheck multiple elements on time then please review the solution provided in the following live example: Checking Multiple Controls
In regards to your second question: The ButtonType = ImageButton will render the spellcheck button as image on the page. However, you want to add a spellcheck button on the RadToolBar control which is different scenario. You need to add a custom button to the RadToolBar, to hide the spellchecker with
ButtonType="None" and in the onclick event of the RadToolBar button to fire the spellchecker with:
var spell = GetRadSpell('<%= spell1.ClientID %>');
the Telerik team