I do NOT wish to combine our medical expenses onto one return. How do I go about overriding that selection?

It's not recommended for spouses to claim medical expenses separately, as you'll actually lose part of the credit that way. The medical expense credit is calculated like so: Your Medical Expenses (Line 33099) MINUS 3% of your net income (line 23600) OR $2,759, whichever is less. If you and your spouse both claim your own medical expenses instead of putting it on just one return, then you will lose that "deductible" twice, instead of just once. 

Also, it might turn out that one or both spouses don't have enough medical expenses on their own to be able to get the credit at all, since you need an amount greater than $2,759 or 3% of your net income.

If you really want to claim your medical expenses separately, then you'll need to prepare your returns separately, as TurboTax Online will not let you split the expenses.