If I have two different fax numbers/lines, but only one fax machine - can I split/merge them, or somehow use them together?
It depends. Does the machine support two input lines? Are the lines 2 virtual 'lines' on one physical line or are there two separate physical lines? Is this at home or an office/home office with a PBX/CENTEX/IPPBX ?
If it doesn't support two lines odds are you need a new fax machine with multi line support (probably very expensive) or run two machines.
Most companies that have a multi line fax need use a fax server since it integrates so easily in modern IPPBX systems.