Often when I stack my fondant cakes, I usually have a small gap between the first cake board and the second tier.To hide the seam I would create a border. How do I get no gap when stacking my cakes. I prefer not to do borders in some of my designs.
You can trim your fondant a bit lower than the board by putting the cake up on something smaller than the board and trimming slightly lower.
Or you can stack while the fondant is still very soft and just smooth it down until it meets the tier below.
In addition to the above,
I have also seen where some (I think it was on ace of cakes) run a bead of royal around the base of the top cake, where it meets the bottom cake, and use a finger to smooth it out so it looks like it's all one cake-just like caulking. I would do this all the time if I had royal icing on hand each week. But I rarely use it.
sometimes I use the "white" wilton chocolate melts ...I melt them put them in a piping bag and pipe around the "seam" then I smooth out with my finger. For off white fondant this works beautifully. I've also done it with other colored melted chocolate to match the fondant.
Thanks for all the responses.