To get this straight... You are building a custom loop, and you want 1 980Ti and have it under a full water block to cool it - correct?
If the above is true its simple. You install the single card, ideally route the tubing to come in/out above but not essential then run as you so wish. If you get another card you simply buy the new card/waterblock and also the bridge to connect the two. Drain the system, install the new card, bridging it to the 1st and re-run the exiting tube from the 2nd where it was previously coming out the 1st to complete your loop again.
Fill up. Leak test. Enjoy new POWER!
Hope that makes sense, its pretty easy to add more is my point - install the 1st card in the upper PCI (usually what you do anyway) then you add downwards (so leave space underneath).