After considering your facts I have come to a conclusion that your query is whether your wife can claim Capital Gain Tax benefit by selling 2 properties i.e 1 Self Owner 2 Jointly Owned. (please correct me if I am wrong).
In that case, yes she can claim the Benefit of Capital Gain by investing in One Property the Sale Proceeds of 2 or more property.
So in your case it will be applicable as the property should be bought either 1 year before or 2 years after the Date of Sale.
Your Date of Purchase of Z1 property is December 2015 and Date of Sale of Jointly Owned Property is expected to be April 2016, which is well within the time limit.
Trust this clarifies your query.
Feel Free to get back/ call for any further clarifications.
CA Rohit R Sharma
BCOM, ACA, LLB - GEN, CERT. FAFP.