not necessary to use a straight shaft with a de-coupler, but a good idea if you want to eliminate that 1/4 of a turn before the front wheels grab, especially if you  are driving on loose sand as the rear wheels will begin to sink before the front wheels begin to grab.
see:
http://www.syncro-bernd-jaeger.de/ENGLISH/Zuschaltb__Allrad/zuschaltb__allrad.html

and
http://www.syncro-bernd-jaeger.de/ENGLISH/Visco-Kupplung/visco-kupplung.html

Also check out the Yahoo syncro groups' archives for MUCH more on this topic.  m2cw  Malcolm

________________________________
From: Jay lefstein <jleftbrane@GMAIL.COM>
To: vanagon@GERRY.VANAGON.COM
Sent: Wed, May 19, 2010 3:34:50 PM
Subject: another syncro question

Hello vanadudes. I was thinking of getting a decoupler for my new
syncro.. does anybody know if I Need to use a straight shaft in place
of a VC in this case?

I think it would make sense to have the DC and VC together but would
be interested to hear any thoughts from others on this

Thanks
J