Not a good system...
The cellular booster and WiFi, both trying to transmit to the same antenna, are likely to block each other at a minimum, or just burn out each others receivers due to the power levels being fed in to them via the opposite transmitter.
Also, by using a single cable to each device, the antennas will act as a single unit with strange directional effects which vary depending on the frequency involved (due to the different signal wavelengths interacting with the antenna spacings).
Depending how the booster works, putting both its antennas in sight of each other may cause it to just lock up or lock in to one way only, like an audio amp feedback howl?
You are far better off keeping the two systems isolated & antennas well separated, and I'd suggest adding an outdoor rated auxiliary WiFi access point, linked to the main one by ethernet.
Also note that passive splitters / combiners typically reduce the signal strength a lot. Plus, cable losses are quite high a the frequencies involved.
Most newer WiFi routers or APs have dual diversity at a minimum using two antennas, or the later WiFi6 units have "beam forming", being able to electronically steer the antenna gain towards each device they communicate with.
Those only work with two independent antennas, or the built-in ones, for WiFi6,