Yes thanks for the precision, i've just seen that now my recovery was TWRP. (well which is good for me since it means I can use OTA). Your details helped me to understand why recovery and kernel are linked together, for my device.
Does it mean that installing omni on a non GS2 device (let's say a more recent device) will not overwrite the existing recovery?
Pretty much all GS1s and Exynos GS2s had this limitation. Qualcomm GS2s (Skyrocket/Hercules) had a separate recovery, as did all GS3s and onwards.