Yes, will be explained in detail with the coming examples. Less than 1 is not possible. A higher Weight Threshold can be used to setup a Multisig Authority Role.

Small Example:

Weight Threshold: 10
Authorities:
[
   [ Alice , 5 ] ,
   [ Bob   , 5 ]
]

In this case both Alice's and Bob's signatures are required to meet the Weight Threshold (5 + 5 = 10). One of them alone would not be able to make use of the Authority Role.

There are still no Steem tools available for working with Multisignature accounts, but I'm on my way to provide Multisig features on SteemWorld in the near future ;)