Yes, you will need to sync them together, but, depending on your situation, simply hooking up your IO to the card may take care of it. Here are some ways to sync them:

If you have Adat lightpipe which is already synced together coming into both cards, the cards will auto-sync to the already-synced signal.

You can run BNC Wordclock to one card, then run BNC Wordclock out of that card into the next card.

Internally, you can use the 2pin cable which comes with the RME to connect the Sync Out pins of one card to the CD in of the other card and it will auto-sync.

You can use multiple BNC Wordclock cables from a clock generator or clock distribution box to run into each card individually.

You could connect S/PDIF out of one card to S/PDIF in of the other.

Adat lightpipe out of one card to lightpipe in of the other.

Just remember that any time you connect something that is sending digital signal (excluding TDIF), it will have a usable clock signal as well, and the card which is now synced to it will also retransmit the clock signal so you can send it to another device. But you don't want to 'Daisychain' more than 3-4 times because the signal could get out of phase and latent.