we’re working on a simple multisig contract which uses generalized account feature of aeternity. contract code is being audited right now
source:
but that’s not enough, we need a management ui for this which will are about to start working on.
regardless of that @hanssv.chain started working on a musig implementation which allows to perform 2-of-2, 3-of-3 multisig transactions based on schnorr signatures:
it’s not documented yet. not 100% sure about the state and he is currently busy with other things.
for n-of-m (2-of-4) schnorr based multisig we’d need a change in the protocol first.
we know multisig is needed and we’re working on it 
independently of that, feel free to share your code!
though I wouldn’t recommend anybody to use multisig contracts which haven’t been carefully audited. we saw how this worked out back then in the Parity horror scenarios which also aeternity suffered from 