The resonating field would be an obvious answer as both the caster and the target has Spell Turning on... in fact it was the first one that came to my mind. But then, the rules suggest that the resonation field is created from two Spell Turning fields, and in this case there is only one.
My second thought was that totaly new effect will occur - You cast True Strike, It uses one level of Spell Turning, bounces back on the caster, uses one level of Spell Turning, bounces... until it discharges the field completely and than takes effect. But hey, that would need... well, 7-10 bounces, would take quite long (relatively) and could cause the resonation itself.
Finaly, the "no friendly fire" effect came to my mind. It would be quite logical as the field guards you against spells from outside, is somewhere around you.
Just thought that somebody already asked that question.
The other idea about the usefullness of the Spell Turning just emerged: Is it usefull against ranged touch spells (or just touch spells)? They have no target in description (e.g. Disintegrate), so it shouldn't be turned back, even when the effect is quite similar to other attack spells with target.