When shooting in such poor light that you cannot get AF, several choices: 1) on a tripod - manual focus. Once the green arrows are bouncing from one side to the other then you are close. Stop the lens down a bit and use a longer shutter speed if you can. this will increase depth of field and the likelihood of getting subject into focus. 2) handheld - I look for a sharp edge or area of contrast at same distance as the subject, focus on that, lock focus, recompose and shoot. 3) using flash - external flash like SB800 has an infra red focus assist. 4) nearly all camera bodies have an AF assist lamp. I normally have it set to OFF because it is irritating when using tripod and draws attention when shooting wildlife but you can switch it to ON in the menu and use it for AF in poor light, lock focus, recompose if necessary and shoot. 5) get somebody to shine a torch at the subject so you can focus, then lock focus, kill the beam and shoot. 6) use a hyperfocal distance calculator, choose aperture and set your focus manually using the distance scale on the lens. The distance scales on both these lenses are not that accurate though.
I use mostly options 1 and 2. When using option 2 remember that your centre sensor is a cross-type and more sensitive than others. It is most likely to get focus in poor light.
Otherwise consider a few primes as suggested by Brian. the F1.8 primes are relatively inexpensive and 3 1/2 stops faster than your 5.6 zooms (11 times more light getting in).