My DC went to a co-ed prep with a single class entry (c22) and I think really my only criticism of the school was that it was too small. Or put another way, any criticism we had of the school - and even a school you love you will usually have a few - could really be traced back to the size and probably would have been pretty much negated by a two class intake.
I agree with someone who said that there is rarely an equal split - one of my DC was in a class where their gender levels were really low, and we were worried it might fall below critical mass, although it did get better. (London, so fair degree of movement). We had some friends with a dc in the year above who, with great reluctance, did move their child for that reason, when it got to a 17/4 split.
In one of my DCs year there were some quite feisty class dynamics, which the school had to spend quite a lot of time trying to contain. But if there were two classes it would have been pretty easy just to reshuffle the classes every year. I also think if there was the constant shifting the dynamics would not have arisen in the first place - it seemed to be a function of being on top of each other all the time.
There is quite an intensity of being in the same class from 3-11, which is not necessarily a good thing. Also, as a parent you are lumped in with the same parents for a 6-8 year stretch and that can be a bit... interesting. The school was billed as very "friendly" - which means lots of parental involvement/drinks/meet ups/everyone invited to the party etc. We actually made some v good friends who we are still in touch with 5 years later, but there are other people I could cheerfully have stabbed by about Y3 and you knew you were stuck with them for several more years.
As well as differing class dynamics in a small year, we noticed quite a difference in the ability levels of both years, which I think is magnified with a small pool. One DC was in a "clever" year, the other DC was in a much less stellar one and coasted spectacularly. Interestingly that DC has found the move to an academic secondary more of a jolt.
I'm not sure it is great for developing your interest, although that may not matter in primary. Both of my DCs were in absolutely everything - football team, netball team, choir, play, largely because with such a small year group you have to be. Its actually v good for confidence and interest levels but also a bit odd not to be able to have your own interests. Neither have shown the remotest interest in drama now they are not forced to do it!