I suspect that there is a critical bug in the latest firmware 1.15.26.
When trying to print with dual extruders and using materials with different temperature settings, the start Gcode M104 is not recognized properly and the tool may not be able to heat up. (Printing starts without heating)
By reverting the firmware to 1.15.23, the same Gcode could be used without problems.
The code described is nothing special.
Example:
;TYPE:Custom
M82 ;absolute extrusion mode
;Start GCode begin
G28 ;home
M104 S270 T0 ; Preheat left extruder
M104 S220 T1 ;Preheat right extruder
M140 S100 ;Start Warming Bed
M109 S270 T0 ; Wait for left extruder
M109 S220 T1 ; Wait for right extruder
M190 S100 ;Wait for Bed Temperature
G90 ;absolute positioning
G0 X-10 Y-10 F3000
G0 Z0 F1800