There’s several different ways this could be answered depending on other factors. But to provide a simple answer, my personal opinion is that 2 hour tasks are too small. I’d recommend combining them into one task of 8 hours. If you need the task to show a two day duration, I’d suggest Fixed Units and the assigned resource having an assignment units of 50%. If there’s no reason the task MUST take 2 days, then also use Fixed Units, assign the resource at full allocation, and let the scheduling tool reflect that it will take less than 2 days to complete. (If you force a duration, you’re manually constraining the duration and preventing the scheduling engine from doing what its designed to do)
Overall, I’d recommend keeping the schedule as simple as possible. Understand what you need to track and set up the schedule accordingly. Meaning, is there a real reason each 2 hour effort must be an independent task? Are they each creating specific key deliverables that must be tracked independently? Does each need different assigned resources? Or does each have different dependency relationships? So there could be valid reasons, but in this case, it’s the same resource assigned and it sounds like you just need to make sure that set of work items is completed. So do it as simply as possible in the schedule.
Both Fixed Duration and Fixed Units will work, the question is what do want MS Project to NOT change if you adjust the task. If it is more critical that the assignment units value doesn’t change, then use Fixed Units. If the Duration is more critical and that must not change, use Fixed Duration. Generally speaking, here’s how I use them.
Fixed Units applies more to project resources. For example, Bob is on the project at 50% and we don’t want project to dynamically change that value to something else.
Fixed Duration applies more towards external events where work isn’t being tracked at the resource level. For example, the phone company needs 2 days to install the equipment after it arrives.