Put a filter on one slit to measure the beams. Done.
Edit: this thinks about a related experiment, where you build an interferometer instead of a double slit.
Now rephrase the experiment s.t. you have to beams serviced from a singular beam. interfere the two beams with each other. If you measure on one beam path (e.g. Pol filter) you destroy the interference pattern.
https://en.m.wikipedia.org/wiki/Double-slit_experiment
Put a filter on one slit to measure the beams. Done.
Edit: this thinks about a related experiment, where you build an interferometer instead of a double slit. Now rephrase the experiment s.t. you have to beams serviced from a singular beam. interfere the two beams with each other. If you measure on one beam path (e.g. Pol filter) you destroy the interference pattern.