74181 ALU not working properly cascaded to add 8 bit numbers
Please show how you are interconnecting the two devices, particularly the carry from the LSB to the MSB device. It looks like that may not be wired correctly.

The 74LS181 doesn't deal with signed numbers, just unsigned integers.  It is up to you to interpret the values in the form you need.

The two devices should be interconnected similar to this: