Hello, tresiba is a long acting or basal insulin, which is used to hold blood sugars steady, even when you do not eat.
Novorapid is a bolus, or fast acting insulin it is dosed to correct high blood sugar levels (correction dosing) and to cover meals (dosed based on amount of carbs and protein eaten in any single meal).
They can be taken at the same time, though not in the same syringe.
For example, your partner takes his usual daily dose of tresiba at 7 am.
However, his blood sugar is 150 and he wants it to be 80. He knows his correction dose of novorapid is, for example, 1 unit of insulin will drop his blood sugar by 35 points. Thus, he will use 2 units of novorapid for correction. He is also planning to eat breakfast shortly and will eat 30 g of carbs. He knows that he needs, for example, 1 unit of insulin to cover 10g of carbs. Thus, he will add 3 units of novorapid for his breakfast, giving him a total dose of 5 units of novorapid.
OF course he will have his own dosing regime, and this is just an example.