Will device-based “wear leveling” equalize access across both SSD partitions?

So long as the operating system you are using supports the trim command then unused blocks, irrespective of their logical placement on the drive, will be returned to the wear levelling pool and will therefore be available for wear levelling across the drive.

It helps to not think of an SSD as the physical drive that you partition yourself, as is the case with a standard HDD. What you "partition" on an SSD is actually the space on-top of another filesystem that itself is on top of the raw block device.

As a rather simplistic view, between you and the actual flash blocks is a flash transition layer or similar that does the wear levelling:

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It is upon this FTL layer that your partitions sit, not the actual flash.

As an example, if you were at some point to delete and repartition your drive it is entirely possible that your partition headers would get wear levelled and could potentially be placed in a completely different physical flash block than they were originally. It is up to the wear levelling algorithm to decide how and where to put any data.