I don't know what kind of LCD driver you're using. It seems that with the IDF's esp_lcd component you can try and set the cs_high_active flag in
esp_lcd_panel_io_spi_config_t.
The SDSPI seems to manage CS by software, so you could modify sdspi_host.c and disable or swap the calls to cs_low(slot) and cs_high(slot).
In theory, this kind of multiplexing of CS should work. You'll have to test if it actually does though because there may be some "dummy" data/clocks sent before or after some SPI transactions while /CS is supposed to be high, which instead of being ignored would then be seen as garbage data by the respective other SPI slave.