What makes the F/A-XX twin-engined tailless fighter controllable with engines out?
Those flying wings even before fly-by-wire (FBW) came along are controlled in the yaw axis by asymmetrically deploying the wing-tip mounted split brakes.*
(Source)
With the engines closer to the centerline the effect of one engine inoperative (OEI) is not as pronounced as on jet-liners with the engines far out on the wings. Even with the lack of a vertical fin, the effect will be less than if the engines were more outboard.
In the Boeing 727 it was discovered that an engine out during takeoff did not have a noticeable effect on the yaw, to the point such planes with close engines had different certification for the V-speeds to allow extra time for the identification of the failure. As well as glareshield mounted lights to indicate an engine failure in a clearer manner.
As you can see in the pictures you have, the engines are close, like the 727. And therefore the standard yaw control via split brake rudders* is sufficient.
As for the F/A-XX, it seems it's still a concept. The second image is also a single-engine UAV.

* The aileron-like device that is split at the wing tips. By creating asymmetric drag, the plane yaws.

Unlike conventional aircraft, truly &quot;tailless&quot; flying wings cannot use a rudder for lateral control as it was absent, so a set of clamshell-like double split flaps on the trailing edge of the wingtips were used. When aileron control was input, they were deflected up or down as a single unit, just like an aileron. When rudder input was made, the two surfaces on one side opened, top and bottom, creating drag, and yawing the aircraft. By applying input to both rudder pedals, both sets of surfaces were deployed creating drag so that the airspeed or the glide angle could be manipulated.
(Wikipedia)