toroidals together

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Take Care...

You'll need to take care that the secondaries are all connected parallel, in phase, as out-of-phase connection of a secondary will result in short-circuit current. You can safety-test by powering the transformer assembly with a 60 W bulb in series with the primary and the secondaries unloaded. The bulb will glow very dimly or not at all if the wiring is correct.
The toroidals are identical, same make, same specs, 15VA with 2 secondaries at 9VAC.

The manufacturer specifies parallel or series connection for the two secondaries on each transformer, and I was just contemplating extending the parallel connection amongst multiple transformers.
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Wire them up in phase and try it. If they get hot with no load, you have a circulating current.
Chances are not good that they will be an exact match.

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the dual secondaries will be bi-fillar wound, i.e. exactly equal number of turns.
You can safely parallel the two secondaries on one transformer.

You can series connect two different transformers. The connection becomes the centre tap.

Two 2*9Vac 15VA transformers can be combined to give 2*9Vac 30VA Centre tapped transformer. The regulation for the series pair will be the same as that quoted for the 15VA transformer.
If the transformers are nominally identical then they can be wired in parallel, but I would still use a small ballast resistor. No two cores are completely identical, even if the windings were done on the same machine.

With N transformers, and a ballast of R ohms, all wired in parallel: the output sees extra resistance of R/N, the circulating current sees 2R between any pair of windings.
The winding resistance of the transformers should be enough. There will be at least 0.5-1 ohms of output resistance when looking into the parallelled secondaries of one transformer.

If the transformers are identical it is very unlikely that there will be a problem. I'd be very surprised if the winding machine miscounted the turns! Any (reasonable) core difference is not going to affect the output voltage measurably either.
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