can I reboot one computer, that has frozen, from another

It depends upon the exact model computers. Some models can be put into Target Disk Mode where the computer basically acts as an external hard drive, so it depends if your computers can use TDM. Then it also depends upon the system versions used by the two computers. The system on the host machine would have to be supported on the one you need to boot. You could then encounter issues if the system had been fine-tuned to work on the host machine but then lacked support for features on the first.

Share files between two computers with target disk mode - https://support.apple.com/HT201462 - "Target disk mode lets you share files between two Mac computers with FireWire, Thunderbolt 2, USB-C, or Thunderbolt 3 (USB-C) ports." - May require OSX 10.10 at least. <-- Apple's documentation, as usual, is updated for the newest computers. Target Disk Mode booting has been around for almost 20 years and does not require OSX 10.10 at least.