If you are using 2 cardioid pattern microphones, you could do something similar to ORTF but with a larger median surface, such as a Jecklin Disk [Jecklin disk - Wikipedia], which can be easily made with foam and a wooden or plastic disk. They’re both simple approximations to the human head approach [http://kemar.us/].
I’ve tried numerous techniques from the “Decca Tree” which produces a good “stereo” recording but has some issues with imaging, to 2 or 3 Omni’s across the front of the ensemble (Telarc’s original approach), to various pickup patterns using a coincident pair microphone similar to the Neumann SM69 [Neumann.Berlin]. Crossed Cardioids at 90-degrees sets up the Hafler Dynaquad playback… which produces a simple matrix with quite a stunning surround sound with 4 speakers [Dynaquad - Wikipedia]!
However, reviewing the 1934 Alan Blumlein UK patent number 394,325 on stereo really nailed it [http://decoy.iki.fi/dsound/ambisonic/motherlode/source/British%20patent%20specification%20394325%20Blumlein%201933.pdf] by using a coincident pair of directional microphones which provide a good image, but don’t always pickup weaker sources outside of the main axes of the directional pickup.
I’ve not tried the Calrec Soundfield microphone system, which essentially performs a vector point analysis in the acoustic space [Soundfield microphone - Wikipedia].
What I’ve recently settled on is a pair of coincident microphones setup in Mid-Side fashion on either side of a Jecklin type disk, which can be easily made with foam and wood or plastic. I call it a poor man’s version of the Schoeps KFM-360 which Jerry Bruck had the clever insights to create using Schoeps microphones [Posthorn | Schoeps KFM 360 Stereo/Surround Microphone System].
Using an M-S type pickup permits varying the directionality and stereo spread of the array and can even provide surround pickup with a bit of simple phase reversals in one mic pair signals. But it requires 4 microphones: 2 Omni’s and 2 Figure 8 pickups. However, down the road, if you have cardioid’s now, you might consider augmenting your mike closet with a couple of Omni’s and a couple of Figure 8 pickup microphones (some microphones do permit setting them to several pickup patterns, so you might check those out too)!
This is probably much more than you need to know right now. But it gives you some technical insights into what’s out there, and how to apply them to various orchestral, chamber music or even soloist performance recordings that capture the acoustics and performers in them, depending on the desires of both the performers and recordists!
Good luck and don’t be afraid to experiment! That’s how you learn what works for you and your clients!