Prova cosi....

codice:
<input type="button" id="upa" value="Up" onmousedown="if (event.button==1) {stopScrollLeft=false;setScrollObjects(document.getElementById('a'),document.getElementById('b'));}" onmouseup="stopScrollLeft=true;setScrollObjects(null,null);">
<input type="button" id="downa" value="Down" onmousedown="if (event.button==1) {stopScrollRight=false;setScrollObjects(document.getElementById('a'),document.getElementById('b'));}" onmouseup="stopScrollRight=true;setScrollObjects(null,null);">