can I hook 2 81012096 cables to one thermostat either with or without a relay? thanks

Yes, as long as you do not go over the recommended Amps of the Thermostat. If you download the User Manual from our cable product page and look at table 5 on page 9 of the manual this will help. Also there is a chart on the product page of length of cable and it's total amp draw. Give us a call if you have any doubt and we can help you check.