> - ST-E2 controller (master)
> - 550ex (slave on A)
> - 420ex (slave on B)

> Do both flash units have to be the same in order for the flash ratio
> settings to work correctly?

No. Provided that the units are operating within their coupling range. But
what camera body are you using? Type A- cameras cannot make use of wireless
auto ratios. See:
 http://eosdoc.com/manuals.asp?q=types

> When I press the self-test button on the st-e2, the 550 flashes first, then
> the 420 flashes. I'm assuming this means that things are working properly.

It means that the communication between ST-E2 and the slaves (550EX and
420EX) is functioning. It does not guarantee that the communication between
the body and the ST-E2. For that, you should activate the modeling flash
Function. If your camera does not have modeling flash, use FEL and verify
that communication is working.

> It would be nice if the E-TTL system looked after this combination
> automatically :)

It does. I shoot macro with a 550EX on an OCSC2, with a slave 420EX and
Vivitar 880AFC.

> I wish I had access to a flash meter, then I'd be able to check the light
> output from each flash unit.

Why don't you use the flash meter inside your E-TTL-capable camera body?
 http://eosdoc.com/manuals.asp?q=FELmeter

Cheers
Julian Loke

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