We both have varying income and expenses. Placing all of our medical expenses onto either of our returns would not reflect our actual situations. We need to de-couple that input.
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.