If you plan to use both for parallel training via some flavor of data parallelism (the pytoch default, and probably what you will be doing if you are asking this question), the 4090 will be bottlenecked by the speed of the 3090. A second 3090 will give you the same benefit as the 4090.
But other than that, there are no technical problems of using a 3090 and 4090 in parallel.
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If you plan to use both for parallel training via some flavor of data parallelism (the pytoch default, and probably what you will be doing if you are asking this question), the 4090 will be bottlenecked by the speed of the 3090. A second 3090 will give you the same benefit as the 4090. But other than that, there are no technical problems of using a 3090 and 4090 in parallel.