Error: ValueError: The last dimension of the inputs to `Dense` should be defined. Found `None`

You have None in the length of the sequence in the second model.

i2  = Input(shape=(None, 104))

You can't flatten a variable length and have a known size.
You need a known size for Dense.

Either you use a fixed length instead of None, or you use a GlobalMaxPooling1D or a GlobalAveragePooling1D instead of Flatten.