Yeah, Germany never, never would allow itself two-front war.
The avoidance of a two-front war was the German's main aim.
1) In 1939 they had not expected Britain and France to stand by their guarantees, taking them to be effete and morally weak based on past experience. Poor diplomatic intelligence really.
2) Weserubung and France/Britain was strategically vital to secure the winter deliveries of iron ore and to prevent raiding by the maritime powers into the Baltic using submarines.
3) Britain and the USSR. Hitler believed that Britain's only hope was to bring in the USSR as her continental ally and chose to strike first. Everyone, Washington and London included, expected the USSR to last 6-8 weeks. A total underestimation of the Soviet State and the people to resist and overestimate of the Wehrmacht's capability, but only just.
4) Britain and the Balkans. Germany conquered Yugoslavia after a coup toppled the pro-Axis Prince Paul. The country fell after less than a 1000 German casualties were taken. The argument that this delayed Barbarossa has been challenged by historians looking at German West-East redeployment.
5) North Africa-USSR. Do you save the Italians with a couple of Divisions and let them go down?
6) December 1941. Hitler does not understand the USA. 'Half negrified, half judaised' was his summation, and it looked like Japan had struck a killing blow. The USN had also been fighting an undeclared war against the KM in the Atlantic for months. I think, by this time, with Moscow looking shaky and the appearance of victory in his grasp...
I am not sure that fighting on two fronts is the issue, but rather making sure the resources are there to do so. This does not really begin to make itself felt until late '42? If you want to look at bad multi-front decisions look at Italy (oh boy) and Britain by the end of 1941. The Germans manage happily for nearly three and a half years, because they had the resources to do so and their victories were generally quick.
How the AI is expected to make that call, I am not entirely certain. If it continues with 'Operation Lemming' and loses half the army attempting to support Spain in Rio del Oro, or even Japan in Thailand then lets, for the moment, assume that this judgement is beyond it. So, 'one at a time' seems a reasonable enough compromise.
K