I have 2 STDL4-4 & 1 powerbar 5.1

The 5.1 is connected thru PAB, 1 of the 4-4 is connected thru s1/s2 (s3/s7 is used for my 3 dosing pumps).
Now how do I connect the second 4-4?

Can I connect it thru the dosing pump, can i use a splitter on the S1/S2, do I need to buy the expansion socket or can I connect it with my expansion box 2?