SEO - Two WordPress websites on same domain name in WordPress

1. Yes
2. Whatever negative means to you. I tend to sayt more then with a single site.

The search engine will basically spokent differ between one or more sites. The spider is just indexing pagest sites. So this can have positive effects while the content of one site pulls / pushes the URLs of contents of the other one.

So the arrangement of the sites in URLs on that domain (URL Layout) and it's contents will have the SE influence,t the number of sites you run on the domain.