dnd 5th – What happens when I transfer twin life?

Assuming that Life Transfer is valid for a twinned spell (see below), the key phrase that leads to this result is the following from the twin spell description:

… you can spend a number of ritual points equal to the level of the spell target a second creature at hand with the same fate.

This specifies that we do not cast the spell twice, but that the effect of the same spell applies to two targets instead of one.

The relevant part of the life transfer is:

You suffer 4d8 necrotic damage, and a creature of your choice that you can see within range regains a life point equal to twice the necrotic damage taken.

The amount the target earns is equal to what you lost. Splitting would not result in each target earning an amount equal to what you lost. Note that spells that deal damage are not divided either, so it should not be any different.

There is an argument to make that the transfer of life targets both the creature and yourself, as you yourself indicated in option 3. According to this interpretation, the transfer of life would not be a valid spell for use with the paired spell.