I have a Leviton HCA02 DHC System Repeater mounted in my garage wall.
It is on it's own 2 breakers.....and it works well. I like the X10 test function.
The house is 5 years old and all electrical wiring was done with home
automation in mind....neutrals everywhere.

Since I have this Leviton device, is there any need to be concerned about having
2 of the Insteon devices, one doing RF on each leg of the 120 mains?
X10 and Insteon languages are both coupled across to the other leg already, right?

I realize that most likely I'll end up having at least a couple Insteon devices
on each leg doing RF and so the mesh will happen with RF as well as
getting phase coupled at the electrical panel by the leviton coupler.
Doesn't hurt to have both, right?

I believe the device you have is specific to X10. It is a signal repeater, amplifier. This usually means the signal is received, decoded, and then reinserted on the other leg. Since the decoding is specific to X10, this device would not see, or understand, the INSTEON signaling.

I think your only option is to use INSTEON Dual-Band Access points and/or other similar Dual-Band INSTEON products, like the 2413.

If others have found a way to get the HCA02 to work with INSTEON. I am sure they will post here and correct me.

I have the same X10 repeater. It doesn't bridge/repeat INSTEON commands at all since it cannot decode them, but it doesn't interfere with them either.

I could certainly put the Insteon repeater in the same metal box
in the wall. There's enough room...I'd like to keep the X10 specific
repeater just cuz....and I'll know that x10 signal coupling cannot
be the problem if I leave it in the wall. Better to have the Insteon repeater
or is it better to rely on the RF repeating? I tend to like the hardware
coupling devices - take care of any potential problems at the lowest level possible.
So both devices can co-exist - just wanna make sure ?

There is an Insteon repeater, the SignaLinc - INSTEON Phase Coupler 2406H. But, most people have chosen to go the Access Point route. With two Access Points, or similar Dual-Band device, you will get a true signal repeat across the two A/C legs, not just a coupling. You can search through these forums for experiences with the 2406H to see it it might work for you. But, I would recommend two 2443 Access Points or 1 2443 and a 2013U Modem.

There will be both an insteon repeater hard wired in the same location as above
as well as RF coupling - unless the 2 repeaters cannot co-exist - but it doesn't look like that's the case.