When reading from the TACH count registers in the MAX31790, is it possible to read from two different counts if two 1-byte read commands are used?

Yes, it is possible for the register to update the TACH count register values in between read commands if two 1-byte read commands are issued. For the registers with two byte values, it is recommended to use a single 2-byte read command to ensure accurate information is received from a single tachometer count, as described on page 39 of the data sheet.