You might be confusing half experience with half share of treasure.
Cohorts get 50% (if run as per DMG) of the treasure that each PC gets. So, if you have 4 PCs and one cohort, the cohort gets 11% of the treasure and each PC gets 22% of the treasure.
However, the cohort gets 11% of the treasure of a treasure for PCs two levels higher than the cohort.
A normal NPC of the same level as the PCs typically has approximately 13% of the wealth of an average PC. After a few levels, the cohort will have about 50% of the wealth of an average PC compared to the 13% that other NPCs two levels higher than the cohort has.
This means that eventually, the cohort will tend to have magical items which make him pretty much on par with (or at least a challenge for) the same level NPCs that the PCs might meet, even though he is still two levels behind the PCs.
So, although the cohort tends to stay 2 levels behind the PCs, he still is about as powerful as some of the NPCs that the PCs meet. In my experience, the better cohorts are sometimes as powerful as some of the weaker PCs, even though in our games the cohorts have lower ability scores and fewer less powerful magical items.
All in all, I doubt you will find that the cohorts fall behind.
As for multiple cohorts, the rules allow for a cohort of a cohort (i.e. your cohort takes the Leadership feat), but some DMs might not allow that. Typically, the cohort of the cohort tends to stay 4 levels behind the PCs, but such a cohort should either be a meat shield, or should not go on really deadly adventures (we have cohorts of cohorts running PC owned inns and temples in our current campaign).