Hello, you will need to create one pos file with all the optode locations in. The most genuine way to do this is by using a localiser to create one based on the real-life locations of the optodes. However, if you just want to conveniently visualise the data, you can easily start experimenting with the most helpful orientation for this by copying and pasting the locations from one pos file into the other, updating the optode numbers in sequence, and off-setting all the optodes in one array by a fixed amount in one dimension (e.g., adding 12 cm to the x dimension for all the optodes in the 4x4 array while leaving the 3x5 array as it is.

Good luck!