Safely wiring two strands of halloween LED string lights in series?
It rather depends what you mean by &quot;connect them in series&quot;.
A string of multiple LEDs designed to run on 4.5 to 5V will have all the LEDs in parallel, with current limiting resistors as required.  That resistance may well be in the control box - white LEDs are intended to run on about 3V.  If you connect two LED strings end-to-end, then you will be adding more LEDs in parallel with each other.  You will need the same voltage, but a power supply that can deliver double the current.
But if you actually do intend to run the two strings in series - the negative line of one connected to the positive of the next, then you will need double the voltage.