Using the TFT screen by only the main board is definitely possible. See also here https://forum.developer.sony.com/topic/658/slow-refresh-rate-streaming-the-camera-preview-to-a-display.
But before writing any instructions or hints I would like to have more information.What environment do you use? Arduino or Spresense SDK? What driver or library do you use? Like Adafruit GFX, TFT_eSPI, NuttX? What is the exact display board? Is it https://www.adafruit.com/product/1743? What is the exact connection? Disclosing the code in use would be very helpful to find issues.
I had it running once with the combination Arduino, TFT_eSPI, ST7789. However it should be no big deal to adapt to other combinations.
Without having further info I guess the pin definition and/or connection has an issue. On the main board you have SPI5.