The Select SOURCETODEST command does what you want. Here it is in a button form for pasting on to your toolbar (see the FAQ on what to do with the XML if you don't know already).
<?xml version="1.0"?>
<button display="both">
<label>Select Source to Dest</label>
<icon1>194</icon1>
<function type="normal">
<instruction>Select SOURCETODEST</instruction>
</function>
</button>