@Timothee-Leblond RepRapFirmware supports multiple motion systems, see https://docs.duet3d.com/en/User_manual/RepRapFirmware/Multiple_motion_systems
I think you may be able to run the first tool and the XYZ motion system in one motion system, and the continuous tool in the second motion system. To use your example, it could be written as (NOT TESTED!):
M606 S1 ; enable forking of the file to each motion system queue M596 P1 ; use second motion system T1 G91 G1 E4700 F240 ; Long distance command to be ran in background G90 M596 P0 ; use second motion system T2 ; Run this with second extruder in parallel to the first long distance command G1 E1.2 G1 E2.4 G1 E3.6 G1 E4.8 M400 ; wait for both motion systems to finish, release motion systemsIan