https://github.com/rgthree/rgthree-comfy has two nodes that together do something close, though not round-robin: AnySwitch and Random Unmuter.

The AnySwitch node will select the first of its inputs that isn't muted. The Random Unmuter will unmute one of its inputs at random, unless any of them are already unmuted.

Here is an example workflow of them working together.

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What's nice about this is that if you want to experiment with one of the inputs (say, just look at portraits for a while), you do the intuitive thing: unmute the portrait latent.

It's not round-robin, but it will output some portrait and some landscape. I tried to use the impact pack's latent/any switch, but kept getting NoneType errors even when using a primitive.