Can you connect multiple photodiodes in parallel and treat it as one?
Yes you may put as many PD’s which are current sources to add together like a PV panel array in series /parallel.

But will the optical sensing work? Not unless they are all blocked   SNR of detection depends on this contrast ratio for all currents combined and 0.5mA/mW received typ. And SNR directly affect detection error rate.

So a good wall of laser light must be defined in terms of threshold for logic levels in current, then converted to voltage with R using a TIA with gain.  Optical gain may be achieved with lenses or series strings of narrow emitters or .
??? So this must be defined in the question with the optical path spacing and length, with the size of the object anywhere typ. from 1mm to 1m in size., as well as velocity or time interval for detection.