1+1 > 2

You obviously have spent much more time looking at apartments than I have, but I have seen apartments which were combined from 2 one bedroom apartments to make a 3 bedroom unit usually sacrifing the dining area. It certainly seems pretty common for someone with 2 children to want one of these units especially if they are favorably priced. Also some people create a 2 bedrooom duplex from one bedroom units on different floors. It is kind of similar to building onto a suburban home - you may prefer to buy a larger home but it might be more cost effective and easier to remain where you are and just add on to what you have.

Conversely, I have seen 3 bedroom apartments where the owners have combined 2 of the bedrooms into a large master bedroom which floor plan does not work for everyone, especially since the price of the apartment does not drop to that of a two bedroom unit.

The length of time that you plan to remain in the apartment is a big factor in deciding if a combination unit is the right situation for someone.

Lobster, I dislike the 3 bedrooms made from 2 one-bedroom apts, in general. I am looking at 2 2-bedroom apts with windowed dining rooms & windowed kitchens - so two 5-room apts or 1 5-room, 1 4-room.
Usually rear-facing in small prewar buildings, but you can get decent light if you are on a high floor.

Duplex combos (not as built) don't work very well, IMO.

I would not do 2 bedrooms into 1 large bedroom, prefer to shop for decent room sizes in original apts if possible.

A two bedroom combined with a studio seems to be a combo that gives a viable (and potentiallyp valuable 3 Bedrom 3 (or 2.5) bath apartment without weird layout/hall/BR size issues.

I agree with NPC10023 that I haven't often seen 1BR+1BR combos that yield a good 3BR

"I am looking at 2 2-bedroom apts with windowed dining rooms & windowed kitchens - so two 5-room apts or 1 5-room, 1 4-room... Usually rear-facing in small prewar buildings, but you can get decent light if you are on a high floor."

Agreed - there are some intriguing combo opportunities in this class - especially mid-block, where the secondary lines seldom generate much enthusiasm and the buildings to the rear are often low (e.g. brownstones).

be careful that your monthlies/psf dont render your apt of less val than comparably sized non-combo units---monthlies/psf are usually higher the smaller the apt w/in a given bldg---combos often result in higher monlies/psf, so where you find 1+1>2

be sure to adjust accordingly