i dont see a problem with having them on the same machine as long as that server does not control any funds.
any bitcoin server which can be used to send money should be on another network and on a machine which you control physically to avoid attacks from your hoster.
I dont understand you

In first sentence you say that there is no problem to keep bitcoin server and web server on the same machine,
in the second you say that bitcoin server should be on another network

bitcoin server which has access to money -> another server which YOU host yourself