As Michael said, it is important that how you want to combine them? If you want to make a new variable by performing a mathematical operation between two or more than two variables, you can use Transform menu and then click on compute variable. Then in this window you can write a name for new variable in Target Variable and do the mathematical operation in Numeric Expression by using Functional group.
If the variables are different data types (there are four, nominal, ordinal, interval and ratio) you need to target a data type to convert one or both of them to. Then you can merge
Another way to approach this is to observe if neither of the two variables is ordinal data you could convert both to ordinal and use in a rank test
There are many considerations - it depends on what you wish to test for or analyze?