Technically ETTL is supposed to compensate so if one flash is more powerful so it should balance out. You won't be adjusting ratios because of that. You will want some difference between you key and fill lights.
That said it is ETTL. A great tool but is subject to many variables. There is a pre flash that determines correct flash exposure. Much like your camera's light meter (NOTE: The cameras light meter has nothing to do with the flash) a bride in a white dress, a groom in a black tux and the bride and groom will all reflect light back differently. Typically the white dress will underexpose and the tux will overexpose.
What are your plans? Studio work? If so and if you have the time to set up I highly recommend setting your flashes on manual. Work on one flash at a time. Set the correct exposure for your key flash and then your fill can be 1 to 2 stops less power for starters. Once you set your exposures they will be consistent all day no matter who is in the frame. Also if you are doing studio I suggest a third flash for rim or highlight. Placed behind and to the side it highlights the hair on the back of the head, etc and gives some separation. You can a cheap manual Yongnuo for that.
So if you can describe what you will be doing we might be able to give you more tips.