Once you've found the right spot for your sub, you can rotate the sub however you like (a sub is a point source that radiates outward). And there is no need to move anything but your main listening position chair (unless you want to rearrange them anyway).
You won't have to worry about vibrations, especially on concrete. First, your sub has dual-opposing drivers so that cancels vibration. Even if you had a front-facing or down-facing driver, you need not worry.
Adding a second sub (located as you described) is always a good idea.
Music area: Magnepan 3.6, McIntosh MC2205 & C48, SVS SB13-Ultra, Oppo BDP 95 and assorted equipment.
Movie area: EMP Tek R5Bi, PSA XS30, Denon X2000, Oppo BDP 83.