Looks like 1/2" @ 3GPM ~ 3.8psi/100'
3.8psi x 125'/100' = 5.7psi, so your 5.5psi value checks out.
Usually when you add a parallel line, I'd expect the pressure drop to be halved. But you already calculated the pressure drop based on the halved flow rate, so it should be about 5.5 psi drop.
You will see some pressure drop at the tees,
PVC - Equivalent Length Friction Loss in Fittings, but it's insignificant.