how can we deliver same changesets to different streams

First, note that you have to accept change sets from a work item into a workspace, and then deliver the result to a stream (i.e. you cannot directly deliver a change set associated with a work item to a stream).
You can do that via two workspaces, as Ralph suggests, or you can use a single workspace, as follows:
Select or create a workspace.
Set the current flow target of the workspace to be Stream1.
Execute the "replace with -> latest from stream" operation on the workspace.
Accept CS1 and CS2 into the workspace.
Deliver the workspace to Stream1
Set the current flow target of the workspace to be Stream2.
Execute the "replace with -> latest from stream" operation on the workspace.
Accept CS1 into the workpace.
Deliver the workspace to Stream2.

Select the "replace

You always use a repository workspace to deliver work items. So this is really easy to do.

Stream 1

1. Create a repository workspace RW1 for Stream 1. RW1 flows to Stream 1
2. Load the repository workspace RW1 and accept the change sets you want from work items into RW1
3. Deliver the changes to Stream 1
   
4. Unload RW1

Stream2

1. Create a repository workspace RW2 for Stream 2. RW2 flows to Stream 2
2. Load the repository workspace RW2 and accept the change sets you want from work items into RW2
3. Deliver the changes to Stream 2