one difference chamaedorea catarctum is a clumper.
while chamaedorea elegans (neanthe bella) is not a clumper.
Now before you call foul, yes neanthe bellas are comonly sold grown in multi plants per pot, however in real life the neanthe bella is not a clumper.

plants of an equal age, the cat palm will be larger.

Hey guys
Thanks for the advice. Several things are working against us though...

First, I had divided them up - and there were babies in each clump, and they came apart pretty easy, either way - so my guess is both were planted in clumps, to begin with.
Second, Size - Different growing conditions, etc - radical variance in size - And to begin with, all of mine were approx the same size - from several sources, so age vs size is indeterminable.

Are there any actual physical characteristics of , say, a single plant, of each?
Thanks

I dont think you have both, if you understand clumping,

I do have both - but see, I cut them all apart to divide them up - I did not pay attention to which were actually clumping, and which were actually clumped together.
So, once again - are there any physical characteristics to tell the two apart?

Can I keep my Parlor Palm clumped?

The difference is when they grow seed stalks. The cat palm has seed stalks that come out like fingers while the neanthe bella palm has seed stalks that come up like small berries. i have both too. My cat palm is much taller but it might be older.