What happens if two persons share spinal fluid from the same Titan?
This scenario has happened in the past and has created additional titan shifters, although the mechanics of this process are not explained in detail and it is questionable whether it could be done again.
~2000 years ago, there were no titans, just normal humans, then Ymir Fritz comes into contact with a worm-like thingy and becomes the first (and at that point) only titan. Upon her death, her husband, King Fritz feeds her remains to their 3 daughters hoping that this will pass Ymir's titan powers on to each of them (which works since titans continued to exist). He then decrees (Ch 122),
My daughters. Give birth and multiply. The blood of Ymir must not perish. If you die, my daughters, have my grandchildren eat your spines. If my grandchildren die, pass their spines from child to child.
While the history gets foggy from here on, the end results of this decree are known.
The descendants of Ymir Fritz become the Eldian people / Children of Ymir whom are distinct from "normal people" due to their quirk of becoming pure titans (mindless titans without the ability to return to human form) when exposed to the spinal fluid of other pure titans or the scream of the Founding Titan
Ymir's ability to "titan shift" (swap between human and titan form at will) was splintered from being her's alone to becoming the 9 titans which necessarily means more than one person can inherit a single titan shifter's powers (otherwise there wouldn't be 9 sets of titan shifter powers, although it is not known if this splitting process is as simple as having multiple people consume a titan shifter)
Titan shifter powers are transferred when a pure titan consumes the spinal fluid of the existing titan shifter (many points of uncertainty, does the titan shifter need to be alive? Ymir had already died when King Fritz had his daughters eat her and his decree implied consuming corpses. Does the inheritor need to be a pure titan? Ymir's daughters were necessarily still human when they consumed Ymir's corpse. Is consuming part of a titan shifter's spine good enough? None of the onscreen power transfers ever show an attempt at "sharing is caring")
Some additional thoughts:
The 9 titans have been more-or-less a thing for almost as long as the titans themselves as noted in Marley's history books (Ch 86) which does imply after the original 9 came into being, no new titan shifter powers were created or destroyed for ~2000 years
The 9 titans can be reduced in number, the series begins with only 8 sets of titan shifter powers as Grisha Yeager had consumed Frieda Reiss merging the Attack Titan and Founder Titan together, this number is further reduced when Eren Yeager consumes Lara Tybur's Warhammer Titan
Despite the 9 titans being in the hands of powerful noble families who were in a constant state of in-fighting for much of history, and then the Marleyians who used their titan shifters as weapons it seems none of them ever gained additional titan shifters by trying to split their titan shifters' powers among multiple inheritors. The reason is never stated, it can be hypothesized that it was never attempted or it was attempted and didn't work, whether because the entire spine is required for the power to pass on, trying to get a pure titan to eat half a living person's spine is somewhat difficult, etc. What allowed for the initial creation of the 9 titans from Ymir alone is still unclear.
In conclusion, multiple people inheriting titan shifter powers from a single person definitely happened at some point, somehow which may or may not work anymore. In the present, it cannot be definitively said what would happen but we kinda need to believe one of two theories, no one has tried it, or, it has been tried before and is known not to work.