It seems that the commands \xrightarrow, etc. in mathtools package don't allow me to put text above or below two arrows (\rightrightarrows). Is there any way to do this with extensible arrows? A crude way would be
\overset{b}{\underset{a}{\rightrightarrows}}but the arrows are not extensible. For that matter, it also doesn't support \twoheadrightarrow, etc.