WiFi Controller system for a motor Relay Output
No wiring the motor's positive line through one relay and the motor's negative line through another relay will NOT give you a 20A current capability.
Then the 15A of the motor still has to pass twice through a relay rated for 10 A.

If and only IF you are able to open and close 2 relay SIMULTANEOUSLY then it might work if you connect the relays in parallel. And then still I would not recommend that. It looks like the control application does not even allow you to
switch two relay simultaneously anyway.

But please have a look at the relay themselves, it is possible that they are rated for 10A at 240V, sometimes these relays can handle a larger current at a lower voltage. So you might be lucky since your motor runs at 12 V.

In case 10 A is really the limit of the relays, you will need a relay that can handle the 15A of the motor. Then you switch on/off that relay with the Wifi relay box. If the new relay works on 12 V you can use the 12 V power supply for the motor to power it. Like this:







simulate this circuit &ndash; Schematic created using CircuitLab