First backup your existing 256 card using the instructions in 4.16. Then use 4.18, put this backup in the top field and the new 2600 backup you downloaded in the lower field to create a new card. Then restore the new backup to the 256 card using 4.17. You will have combined two separate SD cards.

If doing this for the first time adopt a "belt and braces" approach. As well as backing up your existing card, just copy the entire Private (and Sd_sound if you have midi files) folder from the card to a folder on your PC. Then if anything goes wrong you have two separate backups of your original card to fall back on to safeguard your data (check in any audio first to check out afterwards).