The MOSFETs that make up each output may not be exactly identical, so the currents flowing through each one of them when the outputs are tied together may not be equal. The MOSFET with the lowest RDS(on) takes the highest portion of the current and dissipates the most power. So when the two outputs are tied together, the current capability will increase compared to a single channel switch, but it may not double.
When a short circuit is applied, the MOSFETs work in current limit mode and the die temperatures increases until it reaches the first thermal shutdown threshold (at approximately 140°C). Thermal shutdown is independent on each channel if the channels are in current limit mode (each channel employs its own thermal sensor). If the short circuit is removed and the normal load is connected immediately, then the power switches will be enabled back when the die temperature decreases below the threshold (there is a 20°C hysteresis). But because each channel employs its own thermal sensor, the power switches will not turn on at the same time, so only one switch may turn on and that switch cannot allow twice the current capability.