Need help pairing an ST7735 with Mega 2560

I have two ST7735 tft displays that need to be controlled by the same board. My Arduino Nano mans the two lcds separately flawlessly, but the Mega 2560 won't even control a single one. I have two pairs of "SCL/SDA" pins marked on the board, and I've tried both of them in every possible combination. The display doesn't even flicker, as it usually happened when I messed up the wiring on the Nano, instead there's nothing but pure white screen.

Here's my current wiring:

TFT --- MEGA

GND > GND
VCC > 5V
SDA > 20 (marked SDA)
SCL > 21 (marked SCL)
RST > RESET
CS > 6
DC > 7

Any idea what to do?

Problem solved! I simply used:

Adafruit_ST7735 tft = Adafruit_ST7735(TFT_CS, TFT_DC, TFT_MOSI, TFT_SCLK, TFT_RST);

so I can choose the MOSI and SCLK myself.

Edit:
So although it works, the update speed is painfully slow without the hardware SPI pins. So, I’d still be eager to find out why don’t any of the claimed SCL and SDA pins work on the Mega. Any ideas?

Regular SPI uses separate pins for data SDI, SDO
The ST7735 uses SPI with a bidirectional pin for data called SDA

Connect SDA to MOSI. SCL to SCK.

Most cheap ST7735 displays require level shifters on the signals if you use 5V GPIO logic.

David.