Martin is correct in what he has said.
Only one turnbuckle is required for each rigging line, it would be pointless having a second one, they are usually positioned in such a place that is easily reached for performing adjustments. Most control lines also had turnbuckles but sometimes these are hidden inside the fuselage.
When it comes to British aircraft you will find that they used mainly streamlined rigging with adjustments at either end so turnbuckles were not necessary, a check with specs for each particular aircraft will reveal what type of rigging they used.
Rigging - the best part of building WW1 model aircraft

Des.