Is there a way to...

[MadmanOfALeader]

...have multiple derivative Civs? Say, have England either colonize America, Australia, or Canada?

 

[Refar]

Try:
<DerivativeCiv>CIVILIZATION_AMERICA,CIVILIZATION_CANADA</DerivativeCiv>

I did not tired it myself but it might work...

Otherwise you would need to code it in python.

 

[MadmanOfALeader]

Didn't work. How do I do this Python business?