Hello, I'm asking for advice.
I'm using NGINX.
I have 2 domains and 1 website.
First domain will host the website and second domain will just redirect to first domain.
Should I use single certificate for both domains or different certificate for each domain ?
Whatever you fancy I guess 
Technically it doesn't really matter. If both domains are for the same "entity" (site) in the end, personally I'd put them in a single cert.
Hi @hrva07, and welcome to the LE community forum
Are they both on the same server/IP?
Yes, the same server, the same IP address. Website will be running behind the first domain, and second domain will just redirect to the first domain.
Either way will work.