Sharepoint - Can I use the same master page between Site Collections?

If you want to copy your master page to another site collection you can download your master page from the master page gallery and upload it to the the master page gallery of the second site collection. From the root site Site Actions > Site Settings > (Galleries)Master pages and page layouts

If you want to deploy the same master page to multiple site collections using a feature, create a module in Visual Studio, add your custom master page to the module, and create an element manifest something like this:

<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <Module Name="Master Pages" List="116" Url="_catalogs/masterpage" RootWebOnly="TRUE">
    <File Path="Master Pages\MyCustom.master" Url="MyCustom.master" Type="GhostableInLibrary"></File>
  </Module>
</Elements>

The basic idea is to have your site-scoped feature include a module that copies your custom master page from the file system to the masterpage gallery. Since the feature folder is shared, the same master page gets copied to the site collection master page gallery whenever you activate the feature.

The master page can be copied in designer and pasted into the new site collection. A better approach would be to bundle it into a feature and deploy it to the farm.