Measurements are reported of the cross-correlation of spectra of differential position signals from the Fermilab Holometer, a pair of co-located 39 m long, high power Michelson interferometers with flat, broadband frequency response in the MHz range. The instrument obtains sensitivity to high frequency correlated signals far exceeding any previous measurement in a broad frequency band extending beyond the 3.8 MHz inverse light crossing time of the apparatus. The dominant but uncorrelated shot noise is averaged down over $2times 10^8$ independent spectral measurements with 381 Hz frequency resolution to obtain $2.1times 10^{-20} mathrm{m}/sqrt{mathrm{Hz}}$ sensitivity to stationary signals. For signal bandwidths $Delta f > 11$ kHz, the sensitivity to strain $h$ or shear power spectral density of classical or exotic origin surpasses a milestone $PSD_{delta h} < t_p$ where $t_p= 5.39times 10^{-44}/mathrm{Hz}$ is the Planck time.
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