Is it possible to save backups from two machines - a desktop and a laptop in one capsule of an external hdd?
No it is not, there would have to be a share location available which is not an option within the backup capsule.
You are welcome, but just to clarify that wouldn't be possible. The way the backup capsule is created it would defeat the purpose the backup capsule. Only Paragon Software is able to recognize the secured partition and would make it susceptible to virus', malware, etc. if there was a share location created in that manner.