@Nick1735 Rather than thinking of marketing as 2 separate listings, I would have it as one listing and market it towards families who need some separate private space, basically the way you foresee using it yourself.
So Mom, Dad and the kids get the house, and Grandma and Grandpa have their own space to retire to if they want to nap, go to bed early, wake up and start their day at 5 AM (which many older folks do) or get away from the hustle and bustle.
Or the teenagers get the upper garage unit because they stay up til 2 AM, want to listen to their music, and sleep til noon.
It could be other family or friends situations as well, but I think if you market it right, it would be an attractive situation for those who want to vacation together, but would also like private spaces beyond a bedroom in the same house.
It would save some money in that the garage unit could just have a mini-fridge and coffee maker sort of kitchenette area, rather than an entire kitchen, and would do away with the concern about privacy for 2 unrelated sets of guests.