WELCOME to the Forum!!
Fear from something in the past is a task to work on, to overcome and to teach the animal he not need fear it with you...but you will need help to do that as you yourself are not as accomplished as that re-educating is going to need.
I too am not sure why the "need"
If the horse is all those things you just described as a riding partner, then ride him.
Same for the wife's horse...
Lunging puts a lot of stress on the horses body most don't realize.
Over time, done in to small and to quick a pace...it can be damaging honestly.
There is an art to using a lunging session to work, to train, to exercise correctly with control
not just let the horse run crazy in a circle which is what many think lunging is about.
If you just need the horse to run and move its legs for a bit before riding, put him in a large pen and chase him a bit but forget the lunge lesson time.
You are also in a testing period, and everything you do that horse is watching for a weak link to authority and respect given or not...so choose carefully your task that you remain top-dog in authority or be low-man on the totem pole and not respected.
Same for the wife's horse...pick your battles carefully.
That means set yourself up for success and achievement, not a failure.
So, if the horse fears a whip...
Someone may have already mentioned this...
Use a twirling lead shank, just something to make a extension of your arm and movement directed as you wish and want.
But with help
, over time, work that fear issue so you can use, or just carry a whip of any kind with this horse and he not fear but respect...a huge difference.