It could work if you stored the database on the USB. Then as long as you plugged in the USB before you started eM Client, whether on the PC or Laptop it would use the same data, but it is going to be very slow accessing the database from a USB.
You need one license per computer as the application checks your license against the motherboard ID. You can deactivate the license on one computer and then activate the same license on the other computer, but with sharing data from a USB and all the time activating/deactivating licenses, it is not going to be practical.
Best solution:
Use IMAP or Exchange so the data is automatically synced in real-time between the devices, and purchase one license for each device.