Hi,
If I understood corectly, you want to connect the fax to modem in a similar way like you connect two network cards using a cross cable. It is not possible, you have to use a pbx (as small as possible - office type it is enough). Basic functions like dialing, dtmf recognition, line voltage supply for both modem and fax , ring generator, all these must be performed, modem and fax are "slave" devices. These functions are performed only by "master" - a pbx.