I'm not sure where we have got to by now. I am not expecting anyone - well maybe just a few - to be doing this now. Building up a second box from the initial post was to be starting now(ish) so any splitting/artificial swarming will be a ways off yet. It may have been warm the last few days, but up here in Lincolnshire it is decidedly chilly at the moment.
These new queens will need to get mated and they will need a few days of continuous warm weather for optimum mating conditions.
Just not warm enough yet, this year, for me to be confident of queen mating conditions - it can be poor in the Uk even in June! So I am in no hurry yet to be splitting off nucs and the like.
So, Peteinwilts, building up will just mean a further brood box on top of an already strong colony on the single brood - one may not want to put on a full brood box of foundation if the weather is not good enough (packing out with dummy blocks is an option to keep the boxes warm enough). When both the broods are nearly filled is the time to separate them, so the super placed between will have frames in it - they should by then be collecting large amounts of excess nectar in most areas so would normally have, or need, a super on top already. That super, if it is there, can be used to separate the brood boxes, or another if they have already filled one already. There should be few issues of a cold colony when this is done as the colony should be very strong and the weather more like summer than it is just now.
I never like to be prescriptive of when to do anything with bees. There is a large difference in conditions from the Channel Isles to the northern parts of Scotland or even from the west to east coasts. I have bees on single 14 x 12s and some with an extra super for brood and a super for honey. But it is still only a chilly April so I am just letting them build up further and collect some honey. Last year all sorts were swarming or ready to swarm by now, around here, including my bees.
Don't rush it or you may finish up with poor queens being superceded later in the year, along with the problems that may entail, if very late in the season. Any nucs split off will blaze away if the weather is warmer, but may struggle if it is cooler. for inexperienced beeks with few hives it is very important that you don't get your bees weakened by a cold snap and splitting too early. Have I made my point?
Regards, RAB