2nd keyword in a 2nd domain name

My site has two major keywords. Url i have been using in the last 5
years already has one of the keywords.

I just bought a 2nd domain name with the 2nd important keyword.

My purpose is to rank well for both keywords. What should i do now?
What would be the best strategy?
Just forward this second domain to the first?
Insert content in this second domain and then a link to the first?
Any suggestion?

Any precaution?

Regards
NF