Accepting it as legal tender doesn't mean it is what the loans are denominated in. Nobody sane would denominate loans in BitCoin. (There are loans denominated in BitCoin. The obvious syllogism applies.)
It is an existing feature.
And this is significant: debtors could repay their debts when bitcoins look like they will decrease in value with respect to the currency of denomination with an intent to harm the debt owner.
In the 19th century, there were dual currency systems with fluctuating exchange rates. I don't know anything about why they failed, just that they failed...
The loan can't be denominated in multiple currencies at once, as you seem to be thinking of, because that would make the loan amount undefined unless the two currencies were directly pegged to each other.
You can have a loan of 100 dollars. Or you can have a loan of .01 BitCoin. You can pay either off in dollars or BitCoin. How much a given pile of currency will pay off depends on exchange rates. But that doesn't affect the loan itself.
I somewhat sarcastically said nobody would denominate a loan in BitCoin, but not sarcastically at all, nobody would ever denominate a loan in "either this much of one currency or this much of another" because that is seriously a no-win situation for the entity loaning money. Obviously the loanee would pay it off in whatever is cheaper.