You can certainly install hard drives into a Mac Pro without partitioning them. Not only can you have two internal hard drives…you can have up to 4.
I think that there may be some confusion regarding partitioning. Partitioning "usually" is when you take a single hard drive…and create more than one partition on it. What you seem to be asking is…taking two hard drives…and having them act as one single larger hard drive. If this is the case…here's how you do it:
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