I guess there are different schools of thought in this respect. Taking airlines as examples can be misleading;
for instance, SAS use A319/321/340s but then also 737-600/800s. Odd when one advantage of the Airbus FBW family is cross-qualification (one gets signed off for A318/319/320/321, then with differences training, A330/340 as well). Another example, Turkish Airlines: 737-800, A321/A340. In this case the reason might be the lack of available capacity in the 200-odd seat range at Boeing (until the arrival of the 737-900ER), but surprising all the same in terms of overall fleet efficiency. On the other hand, as has been mentioned, Southwest Airlines only have 737s, Ryanair only use 737-800s, easyjet has decided to revert to a single fleet and replace its 737-700s, Air Berlin will replace its (new) 737-800s with an all A320 fleet apparently.
Training costs (flight crew, cabin crew and engineers) are obviously higher with a dual fleet, but there are other, hidden, costs in terms of personnel/rostering (having the properly type-rated crews rostered or on SBY at the right time/in the right place), maintenance (having engineers qualified on one, or both types - with the possibility of having to place separate line maintenance contracts, having a dual inventory of spares), commercial (with the possible overbooking of a type replacing another due to the original aircraft being "tech"), etc.
Then again, at the negotiating stage, a dual fleet might allow an airline to have more leverage on each manufacturer...
Cheers
