It's perfectly fine to do, as long as you know what you're doing.
If the diodes match perfectly, at all times, then it is safe to use the combination at twice the rating of a single part, or however many.
If they don't match at all, like one diode is an open circuit and not even there at all, well, obviously the limiting rating is just one diode.
So for two in parallel, you have somewhere between 1 and 2 times the current rating of a single diode.
Where in that range, is the real question.
For diodes that are closely thermally coupled, it can be pretty high, even without ballasting resistors.
Note that pairs are typically two dies, so although their temperatures track closer than separately packaged diodes, it's not as good as a monolithic pair (two diodes on the same die). Which also exist, though the datasheet may not say so, so it can be hard to tell, short of breaking the thing in half to see for yourself.
Adding ballast resistors, improves current sharing so that the limiting current can be much closer to 2.0 times. This comes at the expense of voltage drop.
Current sharing also depends on diode type. Some are better than others. Typically, PN junction diodes have a strong negative tempco and low incremental resistance, so they parallel poorly. Small schottky diodes (like BAT54) may parallel okay, on account of operating at relatively high current density (internal resistance is more significant, or even dominant). SiC schottky are almost always in the PTC range, so can be quite safe to parallel.
The applied current matters. For all semiconductor diodes (as far as I can think?), the ideal diode junction part of its response, always has a negative tempco; this is always in series with a resistive element (bulk semiconductor), which has a fixed resistance, and positive tempco. At some current, the PTC will always come to dominate; but this point may be well beyond the continuous operating current of the diode, so it's not always useful. Or, perhaps that's exactly the point, you're concerned with surges -- in which case it may not matter very much. (For surge purposes, paralleling of diodes and TVSs is generally acceptable, if maybe not preferable.)
The exact current sharing factor can be calculated from forward voltage curves, thermal resistance, applied current, and ambient temperature; unfortunately I don't have a formula handy for that (if a closed-form solution exists), but fortunately that would exceed the scope of a beginner topic anyway so I'm not on the hook for that, right?...
Tim