I'm thinking that two voltage sources, not producing 'identical' (theoretically) voltages will lead to interesting dynamics going on with the currents (and the voltages) when supplying a load. At least the supplied voltage might be noisier than having just one regulator by itself.
If some extra work is added, like current sensing and compensating one (or the other) regulator based on voltage and/or current monitoring, then could end up with a nicely behaved system.
But probably no need to bother because everybody here explained ----- might as well use a single regulator that can handle the power.