Using a pair of Reach RS will give you the best precision because they are two identical receivers with two identical antennas. However, the next best is to use a CORS as your base station.
Let us say that you can create a map using GCPs and get 3cm precision. Also, let us say that experiments at your project location prove that precision with a pair of Reach RS is about 1cm and that accuracy with your CORS corrections is 5cm.
So, if you use a single Reach RS with CORS corrections, each GCP is then accurate to 5cm, and any point on your map will be 5 + 3 = 8cm precision, and the map will be placed in the world with: 5 + 3 = 8cm accuracy
And if you use a pair of Reach RS with CORS corrections (only for the base location), the base is accurate to 5cm, and each GCP is precise within 1cm relative to the base. Any point on your map will be 1 + 3 = 4cm precision, and the map will be placed in the world with: 5 + 3 = 8cm accuracy
That is a bit over-simplified, but you get the idea. Having a second Reach RS doesn’t align your map to the world any better, but it makes your map more precise.