Another way to do it, assuming that the list can be mutated: find the largest number in the array, store it somewhere and remove it from the list. Do this a second time. Then add those two numbers.

It evades sorting the array, but you have to search it twice.

    a = [7, 9, 2, 8]
    
    highest = max(a)
    a.remove(highest)

    nexthighest = max(a)
    a.remove(nexthighest)

    print highest + nexthighest    

Optimisation: just scan the array once, and keep track of the two highest numbers.

    a = [7, 9, 2, 8]

    highest = None
    nexthighest = None

    for item in a:
        if not highest or item > highest:
            highest = item
        elif not nexthighest or item > nexthighest:
            nexthighest = item

    print highest + nexthighest