suppose a thing has 2 properties a and b. Now i want to generate an alert when a=5 and b=10(if both conditions fulfill then only alert is generated)
You can achieve a similar functionality using the DataChange event for those properties, then after verifying if you condition is met trigger some custom logic (like writing to the log or in a stream). You can't however trigger an entry in the AlertLog.
However, coming back to your question, at this moment this is not a supported functionality in ThingWorx - having an alert depending on more than one property, and with a more complex logic. I would suggest sending a ticket for enhancement request through the support.ptc.com portal.
On the DataChange event of property A, you can check if propertyA == 5 && propertyB == 10 before generate an alert. And the same check for the DataChange event of property B.