I have a website in which the URL structure is
I would like to have another section in the same site with
Does it make any problems? Is it advisable?
Having different styles of URLs for different parts of your website is not an SEO problem. As long as your URLs uniquely identify content and are not spammy, they are fine with search engines.
The only place that I could see this becoming an issue is if you tried to apply rules to clean up your URLs. In that case you might need to write different rules for the different sections of your website rather than apply a single rule site-wide.
Using more than one URL structure in a website may not create a big issue unless you have 100% duplicate contents on all the pages. But URL structure with 'dashes' is always better than 'underscores'
Matt Cutts stated in his old post that URL structure with 'dashes' would be more preferable than 'underscores'.
He gave an example for his statement: