If you want to enable synchronized shifting or semi-synchronized shifting on Ultegra Di2 6870 or on DuraAce Di2 9070, Its possible from now on.
On March 10th Shimano released new Firmware for the front and rear derailleurs. With version 3.0 installed you can enter the configuration menu in e-tube project and set the configuration of the shift modes.
There is only one thing you mght have to change to make it work. synchro shift only works with the newer battery type on road bikes. So if you have the old battery SM-BTR2 you have to upgrade to the newer BT-DN110.
If your system meets all the requirements, its easy to toggle between shift modes. You just have to double press the button on your junction. If you get one blink for confirmation, manual mode is set, if it blinks twice, shift mode 1 is enabled. If it blinks 3 times your bike is set to mode 2.
In default setup mode 1 is synchronized shifting and mode 2 is semi-synchronized