First of all, it is important to know what the involved frequencies are, and why one of them is FSK-encoded, as it might be advantageous to FSK modulate the digital data at the "other end" if at all.

Additionally, it would be useful to know what sort of "line" you mean to use, both in terms of signaling and in terms of protocol.

There are numerous ways of doing what you want, but a digital scheme may serve you in this case, as it requires little analog (fragile and subject to the perils of noise, drift, component-aging, etc.) work.

If you have two initially separate signals that you can sample, digitally, i.e. anti-alias-filter, then convert to digitized format with a codec similar to what's used on PC sound cards (try it with a sound card, if you like) then you can mix those, digitally, by shuffling bits of the digitized signals into a common bitstream and then "unshuffle" them at a the receiver. If the frequencies are both in the audio range, this should be no trouble, and you can easily attempt it with your PC by feeding the FSK into one channel of your stereo stream and feeding the audio into the other.

If this gives you any ideas, then perhaps you can run with this on your own.

I can briefly describe a means I once used for mixing a digitized audio stream into an already digital signal stream the bits of which were at 15x the bit rate of the audio signal.

I used a 4-bit counter (e.g. 74F193) with an XOR (of bit-rate-clock XOR carry-out) in feedback to its clock. This produced 16 clocks for every 15 bit-clocks fed to it. The carry bit steered a 2:1 multiplexer with the two separate bit streams in and one out, such that on that "extra" clock pulse created by the XOR with carry, the analog bit was inserted into the output data stream. This data stream must subsequently be retimed, i.e. its "extra" clock must be removed, such that the serialized output is made regular. That will removed the abbreviated 16th bit, substituting a "normal" 16th bit for it.

At the receiver, the bitstream has to be synchronized and reframed by using a header, but the two bitstreams can be separated in similar fashion, with the exception that the "extra" bit is still normally timed, so that the analog bits are routed outward at 1/15 the rate of the digital bits, thereby restoring the bitstream integrity, and, if necessary, the timing.

If you wish to do this with your PC, as an experiment, just to prove that you have a workable method, the stereo data file can be transmitted via whatever means you choose, and decomposed similarly at the "other end".

RE