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Make sure you have these domain names on different class IP Blocks.
I should let Pangea answer, but I think he meant Class C blocks.
Every website resolves to an IP address. An IP address is like a vin on a car or a zip code that is separated by dots.
123.456.789.01.234
The dots separate the IP address into blocks. The third one is the C block.
If the IP addresses are too close to each other numerically, the assumption that Google will make is that you are getting link love from your next door neighbor (to continue the zip code analogy) or from yourself.