I'd like to think that it's twice as good as a one-party system.
In the same way, the obvious solution to a one-party system is to overthrow the government, whereas with a two-party system, you're supposed to fight within the system... and mostly accomplish nothing, because those in power have overlapping interests.
And who's gonna do that when the government has a monopoly?
What do you mean exactly? This seems a little too vague.
So as bad as a two party government is one party would likely be way worse.
The "external entity" is the people in revolution against the government. This was how the US itself was founded, in revolution against the existing monarchy.