@WaterWorks - all the terminals on the Rachio are low voltage - 24 VAC. Plus the Rachio protects itself from a zone drawing too much current. Odds of “frying anything” are remote (one should never be absolute).
For option 1 - An ohm meter (normally combined with a volt meter) (and I realize I’m skating on thin ice here as in the prior post there is the comment about not much electricity knowledge) could be used to test connectivity between the common wire and each wire in the bundle. There should be some resistivity (not much) between the common and a zone wire in the same bundle. Between wires from different bundles (e.g. common from bundle 1, a zone wire (say zone 3) from bundle 2) there should be infinite resistivity (i.e. no continuity). If there is resistivity (continuity) across wires from different bundles then the common wires are joined somewhere out in the field. Now that I think about it, a quicker test is to see if there is resistivity (continuity) between the two common wires from the separate bundles. My guess is no, and if you don’t have an ohm meter I might just roll the dice and hook them up. If for some reason one set doesn’t work, then we’ll start trouble shooting the wiring.
For option 2 - No. Put all 9 wires from bundle 1 in the 16 port Rachio. Put the first eight wires from bundle 2 in the 8 port Rachio. Hook the relay up to wire 9 in bundle 2 and port 10 (or 11, 12, 13, 14, 15 or 16) on the 16 port Rachio. Connect the common for bundle 1 to the 16 port Rachio and the common for bundle 2 to the 8 port Rachio.
Option 2 is the next to least cost option (a relay is cheaper than the 8 - 16 port delta and cheaper than two isolators) as option 4 is a no cost option, but uses the Rachio intelligence sub-optimally on the two zones that are tied together. But option 2 will also have a relay (small) and wires running between the two Rachios, so it may not look as “pretty”/neat and tidy, etc.