In the 747 (both Classic and 400) 2 packs can be run on a single bleed source, even the APU. Also, in normal operation, bleed sources are not isolated, so all 4 engines supply air to a common manifold.
True statement about the APU while on the ground, and your point about the 747. When the APU is being used pneumatically, and if it's threshold temperature is reached the APU load control valve will modulate close to reduce air load to protect the electrical system.
On some aircraft lectrical loads will be shed automatically during engine start if the APU's threshold temperature is reached.