[RPG] Can you Dash twice on the same turn

You can only take a single bonus action each turn. However, Expeditious Retreat says that:

When you cast this spell, and then as a bonus action on each of your turns until the spell ends, you can take the Dash action.

This can be paraphrased as:

When you cast this spell, you can take the Dash action. Then, as a bonus action on each of your turns until the spell ends, you can take the Dash action.

Or to summarise, on the turn you cast the spell, no further action beyond casting the spell is required to take the Dash action.

You could certainly Dash first to increase you movement allowance, then attack the first mook, then move, then attack the second mook. That's not exactly what you're asking about though, since you want to see the result of the first attack before deciding to Dash.

In the strict rules as written, however, only movement is explicitly allowed to be broken up arbitrarily. Taking the Attack action doesn't "add to your attack allowance" the way Dash adds to your movement allowance.

Implicitly, when you take an action, you are committed to completing the entire action unless you have a rule that says otherwise. "Moving Between Attacks" is one such rule, and object interaction (PH p.190) is another:

You can also interact with one object or feature of the environment for free, during either your move or your action.

By this reading, you have to complete all the Extra Attacks that are part of the initial Attack action before using Action Surge to Dash.