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Following on the heels of this question, I'm hoping that somebody can help me.
If used with a transmitter/receiver, will the Canon 90 EX speedlite still serve as a master unit and fire a 430 EX II in slave mode? Based off this Canon article, the 90 EX can be a master unit if used on camera. I'd like to use it, though, off-camera, to trigger my 430 EX II.
If not, what's an easy way to accomplish this - using another flash off-camera to trigger the 430 EX II - without breaking the bank?
No. It will not work as a master off-camera. None of the Canon flashes can be used simultaneously as a slave and a master. The 90EX is only a master unit if used on the hotshoe, or tethered off-camera with a TTL cable. If used on manual-only radio triggers, there's no way for it to sense it should perform master communication. And TTL radio triggers act as a master, and treat all attached flashes as slaves.
If you are using a 600D, 60D, or 7D or later body, you already have a master built into your pop-up flash. The simplest way to trigger a 430EXII would be to use that. If, however, you don't have this capability, other methods would include using a PC sync cable or add-on "dumb" optical slaves (which may require hotshoe adapters for either/both your camera and flash), a TTL cable, or radio triggers. The most cost-effective and popular solution would probably be to get manual-only radio triggers. They can be found in the US$25-40 price range for a Tx/Rx or transceiver pair. This will only communicate the "fire" signal. You won't have HSS, TTL, or any remote control over the camera (the flash control menu on the camera will be grayed out). But you can fire the flash remotely, and the 430EXII has M mode, so you can set power explicitly on the flash itself.
See also: What should I look for in a wireless flash trigger for a home studio?
Yes, using a 90EX as a master unit to trigger the 430EX II is exactly a use case that it was designed for.
Keep in mind that this is an optical connection and it's not all that reliable or far reaching.
For much more detail and additional non-optical options take a look at this previous question: How can I fire a single canon 430ex flash remotely from a Canon 6D camera?
