There are very few notebooks around that have a display port or hdmi input to use there display for an external device. I do not remember whether these devices offer to output there mouse/keyboard via usb. The Librem14 does not offer this functionality.
To me it seems a bit odd that you’d want to use the Librem14 as an external display/mouse/keyboard for a Librem5. Both run the same operating system and you can use the same software on both devices. The Librem14 is missing a modem though and cannot connect to mobile internet directly and you can’t make phone calls via mobile network from the device.
You can run phoc/phosh as your graphical shell / compositor as on the Librem5 and you can make phone calls via SIP using calls like on the Librem5. You also can run a virtual machine image of the Librem5 software stack in kvm.
All that aside you can connect to your Librem5 and use applications remotely on your Librem14:
- connect to your Librem5 via network (vpn over public network, usb-c ethernet adapter, wifi)
- login via ssh to get a console - lot’s of stuff can be done this way
- use projects like waypipe or greenfield to use applications on your Librem14
Basically the idea to buy a Librem14 to help your Librem5 seems economically and in the view of computing power strange.
What I and lots of other users of Librem5 and a Librem notebook successfully do is:
- use the notebook and if needed connect to the Librem5 to use software or services of installed on the phone (like modem, sms).
- sync stuff between the devices since both are running PureOS
- carry the Librem5 anywhere containing the data and software you’re running on your notebook
- if the computing power of the notebook is not needed, but you need a console, carry the Librem5 and devices for docked mode:
- Lapdock like Nexdock
- usb-c dock to connect to an external monitor and - if needed - input device(s)
All that makes a notebook running PureOS and a Librem5 running PureOS a great couple to work with and to carry around as little stuff as really is needed.