TPS60400 provide twice load when paralleled?

Yes, the available output current will increase.  In addition, the 2 devices should share fairly well since with increasing load current, the output voltage drops, leading to that self-compensation.  So, this ‘droop’ sharing method is essentially built in.

But if you need higher currents, a buck IC as an inverter may be another option, such as TPS62120/25.