Solution
Two serial waveforms cannot be sourced simultaneously.
One potential workaround is to use parallel waveforms. Parallel waveforms can be used in both source and capture waveforms.
A parallel waveform allows for multiple pins per vector. The waveform needs to be written such that the bits are output on the right pins in the right order. When you configure the source functionality for parallel use, the entire waveform sample is sourced on all specified pins in parallel for each vector that contains a source opcode. Use the drive from source pin state (D) for each pin on each such vector that corresponds to the pins to source.