RAM oddities

Slightly technical question.

I got an extra 8gb for my desktop. No matter how hard I try, Windows 10 doesn’t recognize the extra 8gb. Both RAM modules work when in single channel, but when setup in dual channel Windows 10 will only recognize the single one.
Both RAM sticks are ddr3 1600mhz but they’re not the same kind of ram, so I’d usually blame it on this and leave the newer RAM, which has better performance, in the pc and take out the old one.
The odd thing is: while I had both RAM sticks in the motherboard, with only one of them being recognized, I opened one of my editing programs and loaded a crapload of photos. This process used to basically lock my pc for a few minutes (the reason why I decided to add some RAM), but this time it loaded everything in no time.

Is it possible for a pc to use 8gb+8gb of ram even if Windows 10 only recognizes 8gb?

I’m not an expert but check your windows version, I’m assuming you have windows 10 64-bit. Windows 32-bit can only address up to 4gb of ram.

You sure it is not a problem with the motherboard? Sometimes some ram slots will die. You have consulted the manual and putting the two sticks in the correct slot for dual channel, usually slots 1&3 or 2&4.

After inserting both sticks load the uefi (usually tap f2 after starting up). See if the ram is recognised there. If not it is likely a hardware problem. Otherwise it might be a windows related problem.

Are they both recognized in the bios?

You said both work in single channel, means the memory is good.

You most likely have the dual channel setup wrong, motherboards vary on which slots are primary and secondary dual channel slots.

EG1: this PC has 2x16GB DDR4 4333mhz in slot 2 and 4. If I put those in 1 and 3, it would only see 1 stick. - Asus Maximus X Hero
EG2: just checked other PC (yay for glass windows), it has 2x8GB DDR4 3400mhz in slot 1 and 3. - MSI Z170 M5 Gaming

As you can see, different brand, different slots.

Try putting the memory sticks in 1 and 3, and then 2 and 4 and see how you go.