This is one of my first SEO mistakes that made a few years back, and its effects were nothing but detrimental (that means bad). If you have more than one website hosted on the same server and link to each one of those websites from each one of your websites, Google will pull a red flag up after seeing that each outbound link originates from the same server IP. You might say that if you assign a different IP to each account (assuming you have a reseller account), you’ll be in the clear. Google looks at the C-Block of your server IP. So if your reseller account has the IP address of 205.10.123.01, your C-Block is 123. These numbers cannot be individually assigned to your different sites on the server. Thus, impossible to avoid this penalty that Google scouts for.
So, what to do? Well, don’t do it ;). Get each one of your site’s hosted on a completely unique server. I personally love using BlueHost’s shared accounts rather than operating my own Dedicated or VPS server. This way, you’re able to link to your other websites without holding a potential threat to your sites. You also don’t have the hassle of running and maintaining your own server.