Will adding a second sink in a bathroom with only one sink currently reduce the water pressure?
The pressure may be slightly reduced while in use. Most homes have at least 1/2” plumbing and the sink cartridges usually have ~1/8” orifices in the faucet on each side so if you turn all 4 on at the same time you probably will notice a difference but not a significant one. Now a tub faucet that would be a different answer.