If you need a ballast resistor for each bulb - to 'fool' the circuit that there are resistive bulbs fitted - then why not fit two to the same place at the back? It's all the same circuit.
The ballast resistors are actually required because the flasher unit works on resistance. If an ordinary bulb blows, the others on that side will flash faster giving an indication that there is a dud bulb. You won't get that facility with the LED/ballast setup as the resistance will still be in circuit even though the LEDs have burned out. Such is the price of technology.
8 cylinders are twice as much fun as 4