Complex sentence: Meanwhile, Hasdrubal Gisco also advanced his army from Gades into Andalusia, thus Scipio was facing two concentrated enemy forces, one of which would no doubt fall upon his rear if he tried to attack the other.
Split into simpler sentences: Meanwhile, Hasdrubal Gisco also advanced his army from Gades into Andalusia. Thus Scipio was facing two concentrated enemy forces, one of which would no doubt fall upon his rear if he tried to attack the other.