That 3708A looks like a neat and versatile instrument. Amongst its tricks it looks like can do a similar thing with bandpass filters, a power meter and a noise source.
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I have no idea how your RC network work improve the TSC-2-1 port isolation..I think my chinese-made combiner is fine for 100MHz range.
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I can't remember what is inside a TSC-2-1 but I think it may be a magic tee combiner with a 25R to 50R matching transformer at the sum port.
If so, there will be a 100R resistor between the input ports A and B. If there was nothing else but the resistor inside the box (not even a sum port connection) then there would be a 6dB loss between port A and B with 0deg phase shift. This is what you would see with only a resistor in the box and nothing else.
However, there is obviously other stuff in the combiner. If you were to remove the 100R resistor and just leave an ideal version of the other stuff there including the correctly terminated sum port then you would also see a 6dB loss from A to B, but this time there would be a 180deg phase shift.
When you add the 100R resistor then these two A to B leakage paths cancel as they are in anti-phase. Therefore, an ideal combiner would have perfect isolation across ports A and B.
In reality the transformers inside the magic tee combiner won't be perfect. The 100R resistor will usually be fairly close to perfect. However, there will be a sweet spot external termination impedance at the sum port that can re-establish high isolation across the A and B ports. It's just a case of changing the sum port termination slightly away from the normal termination by using a small amount of resistance and reactance. This only works over a narrow frequency range.