I've had a good look at this - I notice in the discount methods, if you choose the 'total item' rule it has the options "greater than", "equal to", "less than" and "contains".
I thought a 'strict' discount method with the following 3 rules could work:
1/ Create a rule 'total item' > 29
2/ Create a rule "total item"
contains [ITEM NUMBER 1]
3/ Create a rule "total item" contains [ITEM NUMBER 2]
My interpretation of the above discount method is that the combined qty of item code 1 & 2 must be >29 in order to apply the discount.
However, when testing, this did not occur (I tried the item number and product ID). Maybe a bug?
Perhaps someone from AC can shed some light on the 'contains' option for 'total item'. (not to be confused with 'contains' option for 'items in the cart')