I am looking at use 2 single slot diffusers attached to a concealed duct mini split. Was wondering if anyone has done this or if this is even right to do. Id like to have a more modern look and not have the grille looked on the wall. 

Is it safe to say that the connection from the outdoor unit to the indoor concealed duct unit is similar to a minisplit in that it is very thin piping and then from the indoor unit to the diffusers, I would need to add some ducting to reach the opening of the diffusers. Is there a specific type of calculation for the sizing of the duct work from the unit to the diffuser? 

If I put the indoor unit closer to the opening of the diffusers, wont I save more money and have less duct work? This is for a new construction so i have plenty of room to play around with.