I get your meaning and indeed a back seat controlling drones and/or EW can be an insane force multiplier, but last I checked the ejection seat of the WSO goes up and not forward, so no a back seat is not a better weapon in that regard. (please excuse the joke, I had to)
on a more serious note, the J20S does exist and should be able to fulfill what you envisioned twin-seaters to do, so I don’t think it’s super necessary for PLAAF to make a twin-seat J35A. Taking the extra space out of fuel tanks means less range which is bad. Taking the space out of the weapons bay means that it probably loses any strike abilities and/or has a smaller A2A combat load. This can be acceptable depending on the circumstance, say you only have the J35A and feel like you need a twin-seat jet to fill certain battlefield roles. However as mentioned before the J20S exists so there’s a better platform (that can take a hit with regard to fuel capacity and still have a huge range) for it as far as PLAAF is concerned.