@seflm The issue you are seeing is normal. By default the I2S hardware is used on the Esp32 to send the data pulses. It only supports one data channel.
You can try the bitbang method at the same time; but I don't believe it will work all that well due to the multi-tasking core the esp32 uses and with no way to suspend task switching (the timing of NeoPixels is very sensitive).
The RMT method is still being worked on and might provide another channel and even possibly 8 channels in the future; but its not there yet.