It's 28. Here are all the probabilities of every possible sum:
8: 5.95374180765e-07
9: 4.76299344612e-06
10: 2.14334705075e-05
11: 7.14449016918e-05
12: 0.000196473479652
13: 0.000471536351166
14: 0.00101689910075
15: 0.00200522024082
16: 0.00365976508916
17: 0.00623952141442
18: 0.0100070492303
19: 0.0151748971193
20: 0.0218430879439
21: 0.0299401768023
22: 0.0391803840878
23: 0.0490493065082
24: 0.0588307089239
25: 0.0676868998628
26: 0.0747718526139
27: 0.0793562338058
28: 0.0809435013717
29: 0.0793562338058
30: 0.0747718526139
31: 0.0676868998628
32: 0.0588307089239
33: 0.0490493065082
34: 0.0391803840878
35: 0.0299401768023
36: 0.0218430879439
37: 0.0151748971193
38: 0.0100070492303
39: 0.00623952141442
40: 0.00365976508916
41: 0.00200522024082
42: 0.00101689910075
43: 0.000471536351166
44: 0.000196473479652
45: 7.14449016918e-05
46: 2.14334705075e-05
47: 4.76299344612e-06
48: 5.95374180765e-07