I personally have never seen a dual system rig with just one PSU. I would go with the dual 750w and call it a day or one 600w and one 850w to leave room for the main rig upgrades down the line.
Ryzen 7 7800X3D, ASUS ROG STRIX B650E-F GAMING WIFI, G.SKILL TRIDENT Z NEO 2x16GB DDR5 DRAM 6000MT/s CL30, AMD Radeon RX 7900 XTX, LIAN LI EDGE 1300, Corsair MP600 PRO NH 4TB