MWBC (Multi-Wire-Branched-Circuit) w/out joined breakers, how best to fix?
Make sure those "adjacent breakers" are in fact not "double-stuff" breakers.  That's especially a problem in GE Q-line panels with their 1/2" tall breakers, where 2 adjacent breakers may in fact be on the same pole.  If each of the two breakers is 3/4" or 1" tall, you are in the clear of that problem.  

Yes, the simplest way to eliminate a MWBC is to nut all the hots together and pigtail them to a single 1-pole breaker.  Now it is 1 plain circuit with much lower circuit capacity, of course. 

Indeed that could be a 1-pole GFCI/AFCI breaker. 

However, for about twice the cost, they do make 2-pole AFCI/GFCI breakers.  These are great for protecting MWBCs and also NEMA 10-wired ranges or dryers.

If you don't want to do that, two single breakers can be tied with a listed (safety certified)  "handle tie" made for that panel and breaker type. These do not provide common trip (much) but do provide the Code-required common maintenance shutoff. Handle-ties are hard to find, much easier to find are 2-pole breakers, at about $9, which do provide common trip. 

MWBCs do not need common trip but do need common shutoff for the reason you now know.    

1-pole GFCI breakers cannot be handle tied to serve a MWBC.  Most 1-pole AFCI breakers cannot be handle-tied to protect a MWBC, but a new type is emerging which can.