Then you will probably find that one of those units (I guess it's the Babylonians) are in fact labeled with merc in the descr_model_battle.txt. That is the Babylonian unit simply uses the merc skin slot on the model as defined in the text file.

So let us assume the code is
Code:
dictionary myunit
...
texture     merc ... .../myunit_merc.tga
texture     romans_scipii ... .../myunit_scipii.tga
And the corresponding edu code

Code:
unit myunit ;; just a name could be anything
dictionary myunit ;; some dictionary unit.
And:

Code:
unit myunit_merc ;; just another name -- doesn't really matter what
dictionary myunit ;; note that this is the same 'entry' as that of the other unit!
Now there's something you would need to keep in mind: the decr_model_battle.txt file (dmb) uses the dictionary 'code'. And as you can see from the pseudo code the two (different!) units refer to the same dictionary 'entry'; hence the same dmb code.

However both edu (export_descr_units.txt) and edb (export_descr_buildings.txt) refer to this 'unit' code. Hence, you can have some edb code
Code:
recruit "myunit" 0 requires factions { romans_scipii, }
As well as some other code in the same file
Code:
recruit "myunit_merc" 0 requires factions { romans_scipii, }
So, the answer to your question? Yes. It is possible to assign both units to the Seleukid roster -- you just have to make sure that the Seleukids (romans_julii) use a similar 'code' setup as the Hayasdan (romans_scipii) do.