Hi there
What I would recommend doing is migrating the old blog over to the new blog so that all of your blog content lives under one consistent URL structure. I would perform both a content audit and backlink audit as well.
Reason being - from a content standpoint, you may have blog posts that can be consolidated, removed, or updated for the new blog. You'll want to run a comb through it to see what opportunities you have there or what hindrances you need to alleviate.
From a backlink standpoint, you may have backlinks pointing to old blog posts that you will want removed if they are irrelevant or spammy, and you may have really good links that you want to update to point to the new blog.
A lot of this will depend on the relevancy and traffic of old blog posts, so make sure you research before you make any moves. If you see equity in the old blog posts, make sure you move those to the new blog so that you can easily maintain and track that content's performance.
However, the most important reason for one blog, is consistency for users and your brand. If you have two blogs providing two separate experiences, you run the risk of lower engagement because it's confusing and not easy on the user.
Hope this all helps! Good luck!