I don't understand your question... DHCP and TFTP have nothing to do with each other. You can set them up on the same machine, or on different machines, with no problems. Is there a specific program or setup that you run that requires them to be on the same server?
I think there should be an option for the dhcp server to specify the tftp server to get the image from. I think there is a man page for the config file, might check that.
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