This post mentions that on three engines, dropping to subsonic, Concorde would lose 30 - 35% range. Much more of a reduction than subsonic planes, but still enough to get to a safe airfield.
On two engines it'll still fly, although with a further reduction in range and possible loss of some hydraulic systems (depends which engines failed).
If I remember correctly, there was some mention of a possible single-engine (plus reheat) go-around, which suggests that at landing weight it might have been able to maintain flight on only one engine.