The plastic tubes are water tube guides... these are to help you align the copper water tubes, to have the water tubes slide inside of these plastic (nylon) guides so that they absolutely slide down into the water pump grommets.

After installation, and the engine running over the coming weeks, months, years, the plastic guides have a habit of sliding/vibrating up the copper water tubes. Look up into that long exhaust housing at the water tubes and you'll no doubt discover the missing one up there, and if so... use some tool that has a hook on it of some kind to pull it down. You'll need both of them inserted into the water pump in order to have both water tubes where they belong.

Without those guides, it would be next to impossible to have the water tubes aligned without going absolutely insane!

EDIT: Forgot to mention..... One guide is a little shorter than the other so that you can insert one tube at a time.

Awesome! I'll bet that's it. Makes sense to me.
If not I'll be back. HA! Thanks man!