Can it therefore be assumed, then, that if light is linearly polarized, it is in phase? More specifically, if two photons, separate from each other, are linearly polarized, are they in phase with each other?

Linear polarisationmeaning

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Two different waves are independent of each other. This means that their directions of travel can be different, their frequency (and therefore wavelength) can be different, the orientation of their polarization can be different, and so can their phase.

Circular polarization

Linearly polarized light is linearly polarized because the x and y components of the electric field are in phase with each other. All this means is that the the x and y components both have the same factor of $\cos(kz-\omega t+\phi)$. For light that is circularly polarized, they have different $\phi$s that differ by $90^\circ$.

Everything I just said was about macroscopic electromagnetic waves. Photons, on the other hand, are not macroscopic waves, they are quantum particles. They are not described by Maxwell's equations. But they do have polarization and thus the same answer applies to them.