I could wire the side port to that second battery. (correct?)
Not directly, there must be a safety breaker in the circuit.
2nd battery is wired directly in parallel to the 1st battery, so both batteries are protected by the big 250A breaker (each battery is also protected by their internal BMSs).
Stock solar controller output (B1 reading on its display) should be wired to a 30A breaker, which is then wired to the big 250A battery breaker.
Your external solar port should be wired the same way, first to a 30A solar breaker, which then goes to the 250A breaker.
Since stock SCC is rated to 30A and wired to a 30A breaker, adding external SCC to the same breaker could trip this breaker if combined solar output exceeds 30A, so the correct solution is to add another 30A breaker between the external port and the main battery breaker.
I tried to draw it in the diagram I found in this thread, hope it makes sense.


