More about invisible links. No, you would not be penalized for multiple doorways on the same server. As long as your domains and IP addresses are different. A minor problem is that you double the chances of a server going down. You now need two reliable servers, but the same effect can be had by using two separate domain names on the same server.
Some marketers keep their doorways on the same server as their site.
I think a separate domain name that has your keywords, is more valuable than a large site. The search engines, like politicians, are an ever changing mass of contradictions. Some information presented here will become out of date before I finish typing this sentence. It requires great effort to keep abreast of these conflicting and secret regulations, and would be easier to follow rules if they were published for all to read, but alas, not today.