Currently Eren Jaeger has the power of the attack titan and the power of the founding titan. Can these titans be splitted again or are those 2 powers supposed to remain like this forever?
There are currently two possible ways to inherit a titan's power:
- Injecting someone with Titan fluid and have him eat a Titan shifter
- Wait for the Titan shifter to die to have a random person inherit it
Going by the first scenario, the person with Titan fluid has to eat the person. It has never been proven or shown that eating just a portion would also grant a portion of the power so there is no way to know. As far as I know, there is no precedent to someone holding more than one Titan power like Eren, aside from Ymir (the first one), so there's also no information as to how to split power from an Eldian holding two Titan shifter abilities. Remember, too, that Eren inherited two Titan shifter abilities from his father (the power was never split).
For the second scenario, as mentioned here, a baby will inherit a power of a previous Titan shifter who died due to the curse. It was never mentioned at all that a baby can inherit two Titan shifter abilities from someone wielding both, only that one of the Titan's power would go to a baby born after the previous wielder's death. (Notice that it's singular, not plural) This is seen in Chapter 88.