This is what I would do, as I am growing some AK48 right now (mine r on day 33 of flower) You can add some soil now and it wont hurt anything (just add enough to support the stem a bit more. You will have to add more light at some point if your growing to grow two plants. I can tell you that mine were 10"-11" when I started the 12/12 cycle and they are now 26" +, these seem to strech quite a bit during flower....something you might need to think about if space is an issue. I am unsure as to what kind of soil your using, but I would wait atleast 2 weeks before giving them anything, I use the Fox farms products (grow big, big bloom, and Tiger bloom) and mine seem to love it. You can go check out my grow journal on my AK's, maybe you will find some of the info helpfull. I just updated it yesterday. Good luck and enjoy the growing.
Stlmatt