Two Macs on 1 Time Capsule

Yes and no.

Yes, if you use Migration Assistant on your current Mac to transfer data from a Time Machine backup of the "sold" Mac that you no longer have.

No, if you are asking if you can use Time Machine to access or view the old backups of the "sold" Mac from your current Mac.

On newer OS the answer is no.. not without breaking several parts of your body in payment. I note High Sierra in your profile.. higher security started with Sierra.

I suggest you buy a USB drive of equal size to the disk in the Mac and download Carbon Copy Cloner. It is free to use for a month. Create a bootable clone of your drive. Then actually boot the computer from the USB drive.

To make things easier in the future I would also recommend you copy all your user files to the share files folder. This makes permissions to access easier from the new computer.

Then you can erase the user in old Mac and sell it. Keep the USB for a year or two until the chance you will need an old file drops below zero.. then reformat it for another job.