How do you sendtwo mono signals in a two channel stereo setup???Welcome to HTS!
It is normally considered better to EQ them both together. [Send them both the same mono signal even if it is a 2 channel music setup.]
If the SW distance from the LP is about the same, say within about 1m, and the SWs are the same model then there should be no requirement to measure them separately.
If the setup is more asymmetrical then it is probably a good idea to overlay SPL measurements of them separately and together to assure the combined SPL is well supported over the bass range.