'Them' in both cases because you talking about both the wrenches and the hammers at the same time. However, if you were actually pointing at the wrenches and hammers and saying that you use the wrenches to do woodwork and the hammers to do housework, then you would say 'those' ie "sometimes we use those (as you point at the wrenches) to do woodwork, and sometimes we use those (as you point at the hammers) to do house work". Does that help?
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.