Normally I would say keep the old Versys, since its paid for. I tend to buy used and keep them until oil runs out the exhaust pipe. My Aprilia was the closest I've come to a new bike, and it was a demo with a few thousand miles on it.
I think you have a legitimate want here, though; more power for long trips, and more comfort for those times when you wife rides pillion. Personally, I am very opposed to debt. What about selling the Versys now while it still holds some value, ride the KLR while saving some cash to throw in, then buying the Super Tenere. You may have to get a used one to avoid borrowing.
Three key parts to a plan like that would be:
1. Getting an emergency fund in place for other household stuff.
2. Getting your wife on board before selling the Versys.
3. Keeping the money in a separate bank account.
You don't want the money to get diverted to something else, leaving you with only the KLR. (not that the KLR isn't a fun bike)