How can I use two output wires as one?
No, you cannot merely tie a differential pair of outputs like this together.

However, you may be able to connect one of the outputs to the ADC input, and either leave the other output unconnected, or connect it to a ground or other voltage reference.   Which is appropriate would depend on unspecified details of the sensor - typically, an active sensor would let you leave an output unconnected, while a passive one would need it connected.   A current mode output could be more complicated still.

Note that using only one side of a differential signal will halve the amplitude, and also greatly increase noise susceptibility - a major point of differential signalling is that added noise common to both sides is cancelled out when their difference is evaluated, but when only looking at one, you see the sum of the signal and the noise.

You are unlikely to get satisfactory results from a load gauge type sensor  using only half of the output, unless there are already electronics between the actual sensing element and the wires you are contemplating.