No, they do not stack at all. Even if you have 18 Scholars in an alliance; only one helix can be on the mob at the same time.
A fresh Helix will always overwrite an older one; regardless of damage, element, or resists (Barring "No effect" magical shields).
Modus Veritas does stack however, and can be applied to any helix regardless of who casted it.
Edited, Mar 15th 2009 4:00pm by Drakonite
It is better wither to be silent, or to say things of more value than silence. Sooner throw a pearl at hazard than an idle or useless word; and do not say a little in many words, but a great deal in a few.