This is one of the reasons why I bought my Scarlett 2i2 interface, so I can access 2 stereo output channels. Channel 1 (Left and Right) is for the speakers being measured, and Channel 2 (with 1/4” jack splitter to L/R stereo RCA outputs) is typically dedicated as the timing reference (using a separate amp to drive a single full-range speaker acoustic reference). It can be done also with just a laptop or the less expensive Scarlett Solo, but i feel it would take more care, coordination and effort to keep the signals routed correctly to the. DUT’s and timing reference.
With my setup I can biamp my mains, or measure a stereo pair, or a sub plus one main, using a consistent timing reference.