A direct conversion SDR (one with an ADC and DAC that covers the entire HF band, for instance) might be able to transmit not only on multiple frequencies, but with custom firmware/gateware, possibly on multiple bands at the same time. But you might have to design a very unusual custom low-pass plus notch filter to make sure the harmonics of all the bands and frequencies used are properly suppressed to stay within legal limits.
A non-custom IQ SDR transceiver (LimeSDR Mini, Hermes Lite 2, et.al.) can transmit on any number of frequencies that all fit under the bandwidth of the IQ data input, possibly an entire HF band, or CW segment thereof.
A non-custom sound card SDR transceiver can transmit on any number of frequencies that all fit under half the audio sample rate or audio bandwidth allowed.
There are lots of articles and web pages on wiring multiple RTL-SDRs together modified to share one clock for synchronous or diversity receive. The TAPR/Hermes line of SDRs often have clock out/in ports for the same for transmit as well as receive.
Or you can do non-synchronous simultaneous SDR QSOs just hooking up multiple SDRs to one console (PC, Raspberry Pi, et.al.). You need to check (spec, benchmark, etc.) to make sure the total bandwidth required (USB, network, memory, page-out to storage, FLOPS, GPU visualization, etc.) do not exceed the max stable load for the system.