Multiple instances of same device on I2C bus
From ADG2128 data sheet:

Every byte requires an ACK.  With parallel devices, the only way you can get a NACK, is if all generate a NACK.  Effectively, you have no way of verifying if the the slave received the data.  You have turned a communication protocol into an unknown.
From Understanding the I2C Bus.

There are several conditions that lead to the generation of a NACK:

The receiver is unable to receive or transmit because it is performing some real-time function and is not ready to start communication with the master.

During the transfer, the receiver gets data or commands that it does not understand.

During the transfer, the receiver cannot receive any more data bytes.

A master-receiver is done reading data and indicates this to the slave through a NACK.



You have defeated the rudimentary error checking of I2C.  Also, you cannot read switch data back without getting a response from two slaves.
The master is blind to slave problems.
You are making ~20 boards to test a product, so I'm sort of wondering why you are concerned about BoM increases.
You do a test with your switch testing your product.  It does not work.  Is it the product or is it your switch?  You cannot do reads on switches to determine if they are in the correct state.
Isn't it better to have a switch board that will work or a switch board that may work?