Booting an iMac using a Macbook Pro's internal disk
Depending on your Macs you might use the FireWire/Thunderbolt target-disk mode.  

Apple-KB:  


  If you have two Mac computers with FireWire or Thunderbolt ports, you can connect them so that one of them appears as an external hard disk on the other. This is called “target disk mode.”
  1. Connect the two computers with a FireWire or Thunderbolt cable.
  2. Start up the computer to be used as a disk in target disk mode:
  - If the computer is off, start it up while holding down the T key.
  - If the computer is on, choose Apple menu > System Preferences, click Startup Disk, and then click Target Disk Mode.
  When the computer has started up, a disk icon appears on the desktop of the other computer.
  3. Transfer files by dragging them to and from the disk.
  4. Eject the disk by dragging its icon to the Trash.
  While you drag, the Trash icon changes to an Eject icon.
  5. On the computer you used as a disk, push the power button to shut it down, and then disconnect the cable.


The target-mode-Mac will act as an additional external harddrive. You may use that as a start volume though and consequently have the MacBook Pro environment with a bigger monitor and better sound assumed the target-mode-Mac is your MacBook Pro.

But the environments on the two internal drives still differ. Example: If you add a user in the above case you will add it to the MacBook Pro System and not to the iMac System.

The only way to have identical environments is to clone one to another.