Is it possible to heat bed and head simultaneously or is it not possible because of the serial structure of GCode?
I do that all the time. I just hit both heater buttons at the same time from Repetier.
It is possible but can be hard on your power supply. The PWM frequency is vastly different between the two heaters. Some have indicated in the past that the much lower frequency of the the bed heaters is hard on the supply and this is compounded by heating the extruder simultaneously. There are mcodes for heating extruders and beds that either heat and wait or heat and release control. To heat both you will want to set the extruder temp with M104 SXXX followed by M190 SXX to heat the bed. This will set extruder temp and release control then immediately set bed temp and wait. Once the bed heats to the requested temperature, control is released back to host. The other issue here is that the extruder heats much faster than the bed and will begin to ooze while it waits. One solution is to heat the extruder to below the extrusion temp say M104 S160, then heat bed M190 S85, the finish heating extruder with M109 S225.
Use this for reference.