Whey from yogurt to curdle milk
If you let the whey sit with the milk at proofing tempreatures (about 42 to 49 C for lactobacillicus bulgaricus, somewhat lower for bifidus, streptococcus delbrueckii and some other strains), you'll get yogurt again. 

In order to get your type of "curds" (corresponding to Russian tvorog, or German quark, or Indian paneer), you need to use acid and heat together. The yogurt whey is a good source of acid, yes. But you definitely have to follow the process as it is, including the heating.