Just posting as a sounding board for this idea:
Figure out the PC's Leadership Score, finding out what maximum level of a cohort he can have. Then, use the Experience Points Awards Table under Gamemastering, from the CRB, to match that level to an allotment of XP of cohorts the player may buy.
That would normally give, considering if the player wishes to maximize both cohorts levels, two cohorts at 2 levels less than allowed.
As an example, a 12th level PC with a leadership score of 15 can have, based on the Leadership Chart, a 10th level cohort. Using the table above, that becomes an allotment of 9600 "XP" worth of cohorts. The player could choose to have two cohorts bought at 4800 XP each, or both at 8th level. He could instead say he wants one at 9th (6400 XP)and the other at 7th (3200 XP), both together adding up to 9600 XP. Heck, the player could even choose to have 3 cohorts, each at 7th (3200 XP each).
Based on this note about Followers under Leadership: "Because they're usually 5 or more levels behind you, they're rarely effective in combat", I would limit your cohorts to being 5 levels below you. So, if you wanted the maximum number of cohorts all at the highest level you could get, that would give you three at 5 levels lower.
At this point, you use the normal rules for cohorts leveling, but using the XP chart, combine their XP together to make sure they never break the total level the player is entitled to.
As just a side note for my rulings on cohorts as well: Cohorts/Followers are not allowed to have the Leadership Feat themselves and I judged closely if they have crafting feats.
Any ideas on if this would work well?