Instinct solar heart rate is syncing with my wife's Venu Sq2 during activity (Walk).

Ok.  Her "broadcast in activity" was on.  Will test tomorrow if it changes.  Come to think of it I was getting an odd message recently before starting the "walk"  It would say something like "HR ext connect and a series of numbers.  I had never seen that before.  

Sounds like you have found the cause. You should be good to go as long as both of your watches have Broadcast Heart Rate turned off.

I did shut mine off earlier today.  Its bizarre.  We've had these watches for about a year and never had this issue, nor did we change any of the HR settings from stock.  Maybe a recent software update turned them on.  Who knows.  We'll see if it is fixed on our daily jog tomorrow.

UPDATE:  Once I shut off HR broadcasting on both watches everything is back to normal.