|Is it the same two computers all the time?|
If so, and you're 100% sure they're both set to DHCP, then it would seem the DHCP server is confused and trying to give them both the same IP. Now since this is coming from a SOHO router I can't see there's a whole lot you can do.
If your router's DHCP server service is capable of doing so, you could reserve an IP for both. Then the DHCP server would only try to give each unit the IP assigned to it.
If your router isn't capable of doing that, I would consider giving one, or both, a statically assigned IP address.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.