For the amateur recordist, or even the professional, this is a very common recording scenario. Choral singing is immensely popular and the opportunities for recording are plentiful.
In a concert, which for a classical choir will not be amplified unless it takes place outdoors, the solo singers are at the front of the stage, and the choir further back.
This is entirely logical because the audience wants to hear the soloists very clearly, and of course see them too.
But in a recording session – session as opposed to live recording – then you can position the singers wherever you want, within the limitation that they have to feel comfortable so they can give a good performance.
So you might indeed consider placing the soloists behind the choir. Why?
Consider this… Suppose you set individual mics for the soloists, then a stereo pair of mics to cover the choir. This will work fine, except that the solo mics are also pointing at the choir, so they will pick up the choir at a significantly high level.
This limits your options when mixing because when you raise the level of a soloist, then the portion of the choir behind them goes up in level too. And also consider that if the mics are positioned optimally for the soloists, then the sound they pick up from the choir probably won't be all that good.
But if the soloists are behind the choir, then the soloists' mics are pointing away from the choir, and the choir are singing in the opposite direction.
This will give you absolute flexibility in setting the levels of the soloists' mics.
You might say that the choir mics will now pick up the soloists. Well from experience, I can say that this won't be such a problem.
Remember that in a recording session, you as an engineer have a great say in where the musicians are positioned. Take every advantage of that.