When MAX19705-8 is switched from Tx to Rx, the TX DAC bus is disabled and the existing DAC data cannot be maintained. If there is such requirement, MAX5864-MAX5865 can be used.
From the MAX19705 datasheet: "Using the 3- wire serial interface or external T/R pin, select between Rx mode to enable the Rx ADC or Tx mode to enable the Tx DAC. When operating in Rx mode, the Tx DAC bus is not enabled and in Tx mode the Rx ADC bus is tri-stated". MAX5864-5865 has a transceiver mode where the ADC and DAC are on at the same time. MAX19705-8 does not have this mode.