Does this existing wiring with shared neutrals need to be corrected?
That's fine, but you'll need to apply a factory handle-tie to those two breakers.  When someone shuts them off, they must shut off together for safety reasons.  

The handle-tie also assures they are phased correctly (on opposite phases), which they must be to avoid overloading neutral. 

This type of arrangement is called a multi-wire branch circuit or MWBC.  There's nothing wrong with it, and nothing wrong with asymmetrical ampacity as long as the neutral is sized for the higher ampacity.  MWBCs have fallen out of favor because of the rise in AFCI and GFCI applications, which they don't always play well with.