I am thinking that in the case of tramadol it was purely a patent trick. In the US we see all sorts of patent tricks to extend exclusivity to the manufacturer. The most famous was
Furadantin----->Macrodantin----> to Macrobid
This gave the company about 30 yrs of exclusivity and major bucks. Tramadol with acetaminophen also has LESS Tramadol per tablet, necessitating people taking MORE tablets for the desired effect and hence selling more making more $$ for the company. They lowered the Tramadol to 37.5mg from 50mg per.
So of course two of these would work better than one 50mg tablet. I always laugh at the .5mg on some of these creative dosage forms. Now really would 37.5 be better than 37? This is to make generics more difficult to make, IMO.

Another:
Darvon ----> Darvon compound-65----> Darvon N---- 100mg----->Darvocet N (now all removed from sale by FDA as "toxic"....that was 50+ yrs on the market!

The most common patent extender is to make a drug long acting...delayed release: (in the MacroBid example above, MacroBid is extended release meant to be taken twice a day instead of 3 or 4 times a day.

Paxil----->Paxil CR which is funny because the action of Paxil is slow in the body anyway, and has a long half life! Paxil CR had another moment of embarrassment as it was closed down by the FDA as not working...it was unavailable for quite a while as it was "fixed".

Another trick is to have the manufacturer hold onto information about "active metabolites"....and then release the new active metabolite after patent expires on the parent drug.

Seldane----> Allegra
Claritin ----> Clarinex