If the page content is only based on the user agent and is not accessible by different URLs then the search engines are only ever going to see one version of the page and that is the version that will be indexed.
But this is also a problem for users. Users don't always search for help (or whatever "instruction" you are providing) using the device that they need help on. It is also impossible to bookmark (or share) one set of instructions over the other; there is no canonical URL for the different instructions.
Intermediary caches / proxy servers can also be a problem. Caches are usually indexed on the URL - which is deemed to be unique. If you have the same URL serving different content, an intermediary cache (place of work, ISP, etc.) could end up serving the same cached content to different user agents. In order to try and get around this issue you would need to send the
Vary: User-Agent HTTP response header.
The best approach IMO is to have separate URLs/pages for each set of instructions and simply link between them. (You can't auto-redirect as you will have the same user/search engine problems as above.) Change the page title and description to reflect the instructions shown.
However, you still need the ability for the user to view the other instructions, for the reasons mentioned above. And you should still provide the ability to bookmark the different instructions by changing the URL (perhaps vary the fragment identifier).