Simultaneous Master & Slave Network on one Nano

Looking to take a value from a sensor, operate on it, and then retransmit the value as a slave. Tried various software I2C implementations but continue to have issues. Anyone done that? Had one bus as master and another as slave?

There are many bad examples for an Arduino as a I2C Slave.
If done right, it will work if the Master is also an Arduino board.
Being a Slave to the normal I2C bus and creating an extra software I2C bus to retrieve data from a sensor is no problem.

What is the Master ? also an Arduino board ?
How long are the wires between the Master and the Slave ?
Can you show the sketch of the Slave and the Master ?

Is your goal to filter the sensor data and act as if the Arduino board is the sensor ?
The main problem is that an Arduino board needs to stretch the SCL clock to run the onRequest interrupt handler.

If you need some distance between the Master and the Slave or between the Slave and the sensor, then it will not work.
If the Master is not an Arduino board, but a device that does not support clock stretching, then it will not work.

AmphenolSensors:
Looking to take a value from a sensor, operate on it, and then retransmit the value as a slave. Tried various software I2C implementations but continue to have issues. Anyone done that? Had one bus as master and another as slave?

If I have understood correctly, you have another Arduino as a Slave which acquires data from a sensor, processes it and sends to Master. It is possible for the Slave Arduino to operate I2C Bus (bring START and STOP conditions) if the Master Arduino has included the following line in its sketch:

Wire.begin(0x09);   //now Master could be addressed as Slave by 'Slave Arduino which is Master now'.

AmphenolSensors:
Looking to take a value from a sensor, operate on it, and then retransmit the value as a slave. Tried various software I2C implementations but continue to have issues. Anyone done that? Had one bus as master and another as slave?

Why not use SPI? Bus wires too short for your project?

AVR USARTs are capable of full speed master mode SPI. Nick Gammon's SPI tutorial covers that and slave SPI code.

Arduino can be an SPI slave using the SPI port and might still have room to master SPI on the serial port or master I2C.

I have two i2c sensors I need to monitor , operate on to combine a value, then appear on another i2c bus as a slave. Sensors are within millimetres of the nano, which is within 500mm of a Mega 2560 master. Need to do it this way as there is an address clash and I can not change it. So far seem to have issues with a software i2c and wire running concurrently.

I have another nano running as a slave on the same master network, as well as a bunch of other sensors.

Look into I2C address expander.

Let's start all over again.

So you have an Arduino Mega 2560 board.
Can you tell which sensors you have already connected ?
Can you show the sketch of the Master and the sketch of the Nano Slave ?

Which new sensors do you want to add that have a address conflict ?

If you answer those questions, then I have more questions.
Are you aware that the Arduino Mega 2560 has 10k pullup resistors onboard ?
Do you use level shifter(s) for 3.3V sensors ?
How long is the total length of all the SDA wires together ? the same question for SCL.

As far as I know, all the software I2C libraries are totally independant of the Arduino (hardware) Wire library. So I assume that the problem has not to do with that, but that the problem already exists with your setup.

My highlights below and I don't expect you to have software for something you're only just asking about.

AmphenolSensors:
I have two i2c sensors I need to monitor , operate on to combine a value, then appear on another i2c bus as a slave. Sensors are within millimetres of the nano, which is within 500mm of a Mega 2560 master. Need to do it this way as there is an address clash and I can not change it. So far seem to have issues with a software i2c and wire running concurrently.

I have another nano running as a slave on the same master network, as well as a bunch of other sensors.

There are I2C address expanders you can buy cheap since they make so many.

SPI bus length can go > 1 meter. You could use the Nano SPI port to connect with the Mega SPI port or a USART run as SPI master (all AVR USARTs can do that) and still have the I2C port for your sensors.

Done it, turned out main issue was one of the sensors not working properly and corrupting the software bus, put another on and its working just fine.

Used SoftwareWire as master on Digital In/Out, and Wire library as slave on built in hardware. So it can operate in both modes concurrently.

Thanks for those who made suggestion, it led me to the solution.