It's not possible in MultiChain 1.0 to write multiple items to the same stream in a single transaction, though it is possible to write multiple items to different streams.
However in MultiChain 2.0 (alphas) you can write multiple items to the same stream in one transaction. Take a look at createrawsendfrom to do this – see the "Publishing to streams" section here:
https://www.multichain.com/developers/raw-transactions/