Connecting high sampling rate DAC and ADC to arduino DUE, timer consideration
If I'm interpreting the datasheet correctly, you might be able to use the Static Memory Controller (SMC) capability of the Cortex M3 Sam3X to attach some 128K x 8 SRAM devices to your Due.  If your ADC and DAC support tri-state outputs then they could be attached the same way as memories (but ignoring address lines as irrelevant), allowing memory-to-memory DMA transfers.  Ie one could either input and store batches of ADC results at 42 MSPS, or could output batches of data to DAC at 42 MSPS, or could do simultaneous I/O at 21 MSPS.  I haven't read the DMA section closely enough to verify that this will work with the M3.

See Figure 25-2 in Section 25.6.2 of the datasheet regarding memory controllers, and Section 26, “Peripheral DMA Controller (PDC)”, regarding DMA.  Also see the eevblog.com page, “Looking for Cortex M3 or M4 with DMA to GPIO capability”.