The power will be divided equally among all spliced sections so the temperature will be the same in both. The total watts requirement of all sections can not exceed the maximum watt rating for the main wire, plug connector, or any type of controller used. Exceeding that could cause tripping fuses, a blowout or electrical fire.

Since you mentioned "under tank heaters;" no heat source should be used under frog enclosures. Attaching to side is OK .