For safety, don’t try a blind two-way sync first. The safest approach is to copy both drives to a third drive large enough to hold everything, then review duplicates before reorganizing. That gives you a fallback if anything goes wrong.

If you do want to sync directly, use a tool that compares file contents rather than just filenames, since identical names may not mean identical photos. On Mac, rsync is built in and is a reliable option, but it requires Terminal. If you want a friendlier Mac interface, a GUI that uses rsync in the background can make this easier.

Whichever route you choose:

• keep backups before syncing
• avoid relying on filename alone
• check for duplicates with a deduplication tool after copying or syncing

So: safest = merge onto a third drive first; most flexible on Mac = rsync or a Mac GUI built on rsync.