With more crystals, it can support more different sample rates, as these are derived from the crystal(s).

The ones with single crystal do not support 8, 16, 32 and 48KHz properly (33.8688MHz is used in place of the proper 24.576MHz leading to much higher pitch on those rates), those cards are cheaped out versions that should be avoided or if you can, add back the missing 24.576MHz crystal to get proper operation with all the sample rates.

What are the two values, and which is the 'missing' one?

On Sound Blasters you see this as well. Early SB Pro/16 models do not have a crystal for the NTSC base frequency. They use the 14.318 MHz clock coming from the ISA bus.
Later cards have their own crystal, so they are decoupled from the ISA bus, and as such do not depend on the motherboard to deliver correct and stable timing (apparently this was an issue on some boards).
You can see them here: Sound Blaster Pro 2 CT1600 Revisions

Perhaps the YMF71x cards are similar: some depend on the ISA bus, some don't.

If the 'missing' crystal can somehow be related to the NTSC frequency, then this would be a plausible explanation.

Edit: nevermind, it was answered already.
Apparently the crystals are used for the ADC/DAC rate.
The Sound Blaster does not do that, it derives the sample rate in 'software': the DSP runs at a fixed frequency, and a divisor is used to reduce its clockspeed to sample rate, where the microprocessor will manually switch to the next sample every time.

33.8688MHz for 44KHz based rates, internal state machines, FM and SB section, 24.576MHz for 48KHz based rates only.

Tiido wrote:

33.8688MHz for 44KHz based rates, internal state machines, FM and SB section, 24.576MHz for 48KHz based rates only.

If I want to use the YMF719 card as a companion to a more capable card only for its OPL3 Synthesis, lack of the oscillator becomes a nonissue though, does it not? It is merely a problem for PCM sampling?

The cards in question are the following:

The former does not even have a solderpad for the second crystal so I guess it's unredeemable, but I suppose I could solder in a 24.576MHz Crystal Oscillator to the second one and the missing capacitors, and it would just work?

What capacitors do I need?

If you only seek "FM daughterboard" then the missing crystal won't make a difference. It starts mattering only in Windows where you might actually play a music file or video or something that uses 48KHz based sample rate.
The first card will need a more elaborate mod to get 48KHz support, the second card needs the crystal and probably some resistor needs to be removed that will bridge 33 to 24MHz input (chances are the 24MHz input is not connected at all, in which case 48KHz stuff will just not play anything at all and in worst case freeze the card).