When you target multiple creatures with the Blindness/Deafness spell, can you give some deafness and others blindness? Or do you choose an effect once, and all targets of the spell get the same effect?

You can (probably) choose separately for each target

The blindness/deafness spell description says:

You can blind or deafen a foe. Choose one creature that you can see within range to make a Constitution saving throw. If it fails, the target is either blinded or deafened (your choice) for the duration. At the end of each of its turns, the target can make a Constitution saving throw. On a success, the spell ends.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, you can target one additional creature for each slot level above 2nd.

Upcasting the spell lets you target additional creatures with the spell. And if the target fails the Con save, you choose whether it's blinded or deafened. Barring wording that specifies that you must make the same choice for all targets of the spell, my interpretation of the description is that you can make the choice separately for each target, so all targets need not suffer the same effect.

This matches Jeremy Crawford's (unofficial) ruling on a similar question regarding the Twinned Spell metamagic option, which lets you target a second creature in range with the same spell:

Twinned Spell. Chromatic Orb: can you pick another damage type for 2nd target? Polymorph: can you pick a different beast ?

Twinned Spell doesn't require you to make the same choices for each of the spell's targets.

In the absence of specific wording requiring you to make the same choice for each creature targeted by the spell, I think it's reasonable to rule that you can make a different choice for each target.