Hi James,
I haven't use OD Cost Matrix, but if I get this right, you want the user to determine a time and then use this time as a variable in OD Cost Matrix. If that's the case you can just use a list and the user will select the proper time(in model properties-> parameters tab, set a list with the two values for the string parameter)... no need of booleans...
If you insist following a more complex way... this should look like this:
Set the two string variable (in model properties-> parameters tab, set the parameters as optional). Then do what you have already done and use merge branch with the two booleans as input. Merge branch output is the user input and can be used as a variable in od cost matrix.
But ... there is no need of that.
Greetings,
John