You can activate stream 1 and the leads will receive all the emails of the first stream. However, it depends if you want to transfer them to stream 2 after they get the whole group of emails of stream 1. If you won't have any content in stream 2 instead of transferring the leads they will stay in stream 1 and be "exhausted". It depends what will be the transition between the streams.
Hi Alex -
To add to Ronen's reply - once you have members in stream 1 (regardless of whether or not the cadence is set for that stream) and transition rules in stream 2 that pull people from stream 1 to stream 2 - those transition rules will move people to stream 2 regardless of whether or not there is content there. Those people will just sit in stream 2 until you activate the stream 2 content and set the cadence.