Dude, I'm sorry it's always me replying... As my knowledge really isn't all that... But I seriously think, now you mention it, that those pots are WAY too small. I would transplant those babies into their final home as soon as possible. Otherwise you might get a bit of stunting as they try and live in the cramped spaces.
Also, with the FIM, what's your aim with that? Would you not be better just topping, and resulting in 4 main colas?
If you're seriously intent on upgrading the cab, then my first step would be to find two decent sized containers that will fit into the new space and maximise the amount of soil you can get under them. Then just rig up some contraption to keep them lit whilst you're DIYing the cab ready for them.
Now, assuming your bigger box can handle a scrog well, I would hold off doing any LST etc until you've got the screen sorted.
Sorry, I'm rambling, but you've just caught me after my morning coffee, so I have much to say, and many points to digress on... Try and follow me, I know it's a nightmare.
If you topped them both now, that would leave you with four new 'main stems' on each plant. Then transplant them to big containers that will fit snuggly into your new cab. They're still short enough that if you put the scrog screen about 6-10 inches above the soil, then they'd still have time to grow up to it, at which point you just start filling the scrog by daily checking anything above the screen and just dipping it back under. Then assuming you keep her in veg a while, you would be filling the screen with 4 main stems per plant, which would pack out in no time. Then by the time you go to flower, you've got a lovely bed of stems, which will all magically turn into budsites. Leaving you with a thick carpet of beautiful buds, rather than two big bushes of budsites at mixed hights, which I reckon is what you'd end up with if you LST them.
Also, if you do go scrog, I would recommend making the screen out of some nylon line, like the sort you use in those garden trimmers, as it's easy to work the plant around, it's also simple to remove come harvest, and it's not going to cut into the stem at any point as it's got a good bit of give.
Okay, ramble over, hope there's something helpful in there.
mewk.
::edit::
Damn sexy! Are those 5Gallon pots?
Just out of curiosity, what is the yield off of that?
I think they were 3gallon plant pots. Nothing special.