Think about what has to take place in the SV cooking process. Low temperature heat from the water bath has to penetrate your roasts’ thickness. At 133F heat difuses relatively slowly into meat. Not at all as quickly the heat of your grill or smoker.
Cooks calculate SV heating time using meat’s thickness. Length isn’t considered because heat always takes the shortest path (thickness) to the core achieving temperature equilibrium, That’s the point at which meat can be thought of as being just-cooked. After that length of time cooks often add more resulting in aditional tenderness. How much more is up to the cook to decide based on personal preferance and experience.
There’s a particular challenge in SV cooking roasts the size of yours, the length of time can be quite long before achieving temperature equilibrium. You may wish to consult Douglas Baldwin’s work to gain a more complete understanding of the food safety challenge.
Costco sells mass produced beef that’s been moderately aged. Thus i would consider no less than a 10 to 12 hour cook.
If you can afford the time and effort cook one roast as you plan and judging from the outcome adjust the second roast’s cooking time if necessary.