You could use round pen panels or gates. 16' to me is a great size for having a spare around just in case.
My drive through gate is never wide enough it seems when the hay delivery arrives and the hay is hanging somewhat over the sides... there goes my gate... taking it off again. Yes, I partly blame the hay truck driver as he doesn't center the truck through the opening...but man... when I redo my fence posts we are
getting a 16' gate opening!!
OK... if you need to completely separate them from contact you can "stack" those panels if need be. Also you could use from Tractor Supply the agricultural panels for goats, pigs, sheep...livestock in general. They are mostly 5' high & 16' long, fairly rigid made but I would not use it by itself on the bottom without reinforcements of lumber to safeguard your "cranky" horses at feeding time. I would use them for the top separation piece with no problem... just need something sturdy on bottom to support and compliment the "wall".
The panels from Tractor Supply are I think very
reasonable in cost.. doesn't hurt or cost anything to take a look and see if they meet your needs.
That would be my "temporary" quick fix... good luck