There might be systems that operate 'double shift' learning, but very few it successfully. It's normally only done in an emergency, for example during a war when a lot of your teachers have been killed or displaced and you might have a lot of refugee kids as well.
That's because 6 or so hours of teaching time is (more than) a full day of work for teachers with prep etc- teachers burn out really really quickly on double shift and the learning outcomes for children are awful. Even if you had half the number of kids you're looking at a much longer day and a huge proportion of primary teachers have primary aged kids themselves.
To enforce social distancing I suppose they could, for example, have the TA take half the class but I can't think of any schools around here which have the physical space even if they gave up their playing fields. And then you've got a huge equality issue where the children who need extra support won't get it and so it would, I expect, be subject to legal challenge pronto.
Don't forget the aim is not to stop people getting it, it is to stop too many people getting it at once. In that sense it makes sense for the population you increase exposure in first to be those least at risk (with measures for vulnerable teachers and pupils).