First you have to open up the console and type: sv_cheat 1 (this allows cheats) and then sv_portal_placement_never_fail 1 (this allows you to place a portal almost anywhere, specifically on metal surfaces.)
Exit the console menu and shoot a portal on a wall near the cube tube and then enter the console again. Now you must type impulse 101 to allow you the Half-life 2 weapons, and then type noclip and fly above your new mistress.
Shoot your second portal on the wall next to the new companion cube and settle down on an invisible ledge above the cube and deactivate noclip.
Once you have done that, switch to your gravity gun so that you can more successfully grab the cube and propel it through the portal. You can either hop through the portal you've made inside the tube, or you can noclip back to the floor. Now you have two companion cubes.
The easiest way to tote both of them around is with the gravity gun.
But it would seem your bohemian ways can't last for long, as GLaDOS asks that you sacrifice one to appease her.
But hey at least you still have your mistress. Or did you sacrifice your wife? You sicko!
And just when you think you are home free...
I have yet to find a way around destroying both of them. I've experimented with trying to spawn extra companion cubes but I have yet to find the right command. I think it is possible because there should be a code representation of pretty much anything that exists in the game, so one stands to reason that a companion cube could be spawned.
