Two very similar species, sometimes the difference is obvious, sometimes not.
They can each be smooth stemmed, oval, have epiphytes, bend easily or lose their fronds.
The very best way to separate them all, but these two particularly, is by slicing the stipe.
One is usually very sticky with mucus, the other not.
The internal lozenge is quite different in each.
You need a knife, a thin section, camera in one hand, slice in the other - click.
Have fun!