What grammar makes 同 品 項 第 2 仵 6折 mean "Buy one, get the second at 60% of base price"?
I am pretty sure that you have the second to last character wrong. It makes much more sense as 件 (item).  Also, 折 should be read as zhé in this context.  The character has a large number of meanings, but in this context it means discount (and the single digit numbers preceding it count by 10%; this is a common idiomatic construction for expressing discounts).  Finally, 第 signifies that the following number is to be read as an ordinal (1st, 2nd, 3rd, ...) 

So, a very literal translation of 同品项第２件６折 would be:
same product, second item, discounted to 60%