This is only a too long comment:
You have got a perfect answer from user Alexander Samual Dunnett for the presented question. Unfortunately the question didn't present what you want. User joojaa guessed it and wrote a warning.
The comments revealed that you want to combine 2 grayscale photos to get one which is a substitute for a photo which was taken with longer, say double length exposure.
We do not have a slightest idea what camera system you had when you took the photos and what mangling the photos have got between the exposure ending and being "ready to be combined in Photoshop". Only summing the original unmangled data can be considered as something you expect. Revealing what there originally was collected is impossible if images are processed to good looking 8 bit/channel usual photos - a vast amount of original info is thrown overboard without asking because it was probably considered to be only noise - too weak to be interesting - and non-linear dynamics compression has mixed the rest together in a non-reversible way.
The only way to extract something which is now invisible is to start from original RAW images which have got no processing. That's the data what you have.
To get what you expect you need other software than Photoshop as said by joojaa and Billy Kerr - the latter even picked one freebie (=DeepSkyStacker) which at least knows how to use RAWs. Unfortunately I do not know does it solve your current problem.
For more help contact people who have worked with the same problem. They very likely can be found among astronomy and astrophotography hobbyists. Of course also some of us here in GDSE are able to present things which are normally invisible, but they make art.