Hey Mick, THIS is the perfect window to which you should separate the twins and transplant into separate containers. At this point, the roots are not fully developed and can still be rung out of the soil. Just don't wait too long as both root systems will start to entangle and separation of the twins would be nearly impossible without severe stunting.
I did this same process in my rockwool cubes, and I too was a tad bit stoned I must admit. One point I have to somewhat disagree on is what was mentioned about one plant becoming the runt of the pot. From my experience, if both plants are from the same strain, then growth and nutrient uptake rates would be the same if not very similar. The only case where I can see this happening is if you have a high-tolerating nutrient strain, versus a strain that doesn't show rapid growth until 1month and 1/2 into vegetative growth (and only in the case of THE SAME STRAIN, if phenotypes are at EXTREME opposites).
Otherwise the MAIN reason I don't grow twins from seed is the fact you never know if one of them will be male or not. And by that time, the roots are already established as a whole system, and removal could kill the plant. However, starting two clones in the same pot is fair game for all