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The table indicates that OTA2 will require nearly 640K RAM.
For devices that have both internal+external Flash, the DCT can remain in the internal Flash(and execute from there) which saves you 16K. But still, the amount of RAM required turns out to be more than 256K. The internal+extrenal flash you mentioned is sufficient to support OTA2 but the lesser RAM won't serve the purpose.
The OTA2 doc talks about the FLASH requirements in section 5. If a device meets the memory requirements for OTA2, you can try modifying the files as mentioned in section 8 of the doc.