yeah i would start filling up the tank with the water you take out during water changes from the 10g then fill rest with reg water and then put 10g filter on the 40 (or at least put the media from it into the bigger tank's filter) and if they have same substrate then put the substrate from the 10g in there as well, you may have mini cycle but just keep an eye on it and if anything get higher than it should be do a water change Good luck with them!