Is there any issues with using two salt mixes in the same batch of water? I have. A partial water change of tropic Marin and the same of oceanic and would like to get them used up so can I mix them together?
I have mixed both of those salts personally with no issues. My first tank water was Oceanic, but salinity was low and I had Marime so I mixed nothing bad happened.
It's not going to hurt anything. Even if the two salts' params are drastically different from each other. What's the most you'll be changing? 10%? 20%? In your whole tank, that's not a large proportion of "new salt".