Happy to help.... just was concerned that I might lead you down a path of breaking things rather than upgrading!
You can't run unmatched processors. So, best thing to do is to swap out the single current processor and place in your SLAEJ in its place. Once you have proved that will work fine, then you can add another SLAEJ. That -002 heatsink has excellent cooling capacity and getting another one of those will be best because it gets you the newer fan type. However, if you can only find a -001 version or one for the xw6600/xw8600 that will be fine too.
There is a very specific way the current processor sits in its socket, and if you replicate that orientation with the next one you'll do fine. Set it down gently, and flat. I put on the Noctua thermal compound before I set it in place..... evenly spread thin, just thick enough you can't see through it but not by much.