Hi Holly,
Good question, Holly. As it happens, the FM-90X has two output jacks, wired in parallel, one designated as "main" and one designated as being for a tape recorder. They are wired directly together internally, so you could connect one of those jacks to one channel and the other to the other channel.
If the component you are connecting it to includes a mono switch, though, you would just have to connect to one channel, while selecting mono mode. In that situation it would be preferable, although perhaps not necessary, to put a shorting plug on the input for the other channel.
If the tuner had not provided two output jacks, and the preamp or other component it would be connected to does not include a mono switch, you could use an inexpensive y-adapter to connect the single output jack to two input jacks.
A potentially important point, however: I don't see a preamp listed in your system description. What component would the tuner be connected to, and what is its input impedance if you know it? Vintage tube tuners, including the FM-90X, were generally not designed to drive the relatively low input impedances of many modern solid state components. If the preamp or other component the tuner would be connected to does not have an input impedance of at least 47K, and preferably more, perceptible deep bass rolloff may result if just one input channel is being connected to. And if both input channels are connected to, the load impedance seen by the tuner would be half of the specified input impedance, doubling the magnitude of that potential issue.
Also, the FM-90X does not have the kind of antenna connections that are typically provided on more modern tuners. Although it is designed to be electrically compatible with either 72 ohm coax (75 ohms should be close enough) or 300 ohm twinlead, the connector is designed to mate with narrow metal pins that are soldered to the end of the cable. You may want to ask the seller if the tuner would come with a means of mating to that; otherwise you would probably have to obtain and solder suitable pins onto the ends of your antenna cable.
Best regards,
-- Al