But she didn't see two lots of money.
She possibly saw ONE lot of money when they OP put a £50 note on the side table.
The OP's DH says he also left a £50 on the side table, which obviously wasn't there when the OP put down another, she assumes the DH is not mistaken or fibbing, and thus the cleaner took one note and later, took the other.
Who knows if the DH did put it there. Who knows if it wasn't blown off the side table and is now down the back of the radiator. Who knows if the cleaner picked it up, then saw later on that there was still a £50 note on the side table, and picked it up again.
If she picks up money from counters and side tables multiple times a day, most days of the week, it is easy to have a memory slip like that - ask anyone who does something like that every day, because you don't know all the time if you're remembering that days action or the previous days action..
The obvious thing to do is communicate with one another, something MumsNetters seem loathe to do!