I think I know what you're asking for, this company sells several different models to fit gates with a device that allows 1 to 6 padlocks, and it only requires removing 1 padlock to unlock the gate. I seem to remember a technical name for this, multiple custody or something, maybe someone else will remember.
In the mean time check this out:
http://www.sharelox.com/ click the different model numbers at the top of the page and allow time for pics to load.

You may also find this interesting reading:
http://www.schneier.com/blog/archives/2 ... _lock.htmlAnd here are some Flickr photos of what you're describing:
http://www.flickr.com/photos/mikebaird/4349729207/As described on the Schneier on Security site, you could also use several lengths of chain and have a padlock every so often, but in practice I have seen users lock out other users by not using the system correctly.
Hope that helps,
Squelchtone