I think the 1/4 will work for both but i could be wrong?
you can buy posts for for the sankey in both 1/4" and 5/16" they just screw on.
there are some conversions around ... cant find the link im looking for.
here is one.
*Components:Sanke Commercial to Homebrew Keg Conversion
there is some around with quick disconnects to make it easier.
or change your gas manifold so that it has 2 different size barbs. - im new to keging myself but that where i would start looking ...
someone who knows more should be along soon.