Edit: All the breakers are 30A capacity. The main panel has 125A main breaker while the Siemens PL subpanel has built in 100A main breaker and 6 pcs 30A GFCI 2-pole breakers. If I use 3pcs 60A breakers for the feeds from main breaker. The current would divide among the 3 pcs, right? The reason most people use mainly one breaker as main feed is because the are no more spaces in the main panel. But if you have spaces, you can make it redundant by using 3 breakers as main feed, right? From an electrical standpoint, nothing wrong with this, isn't it?
I'll replace 6 of the 16 2-pole branches with 6 GFCIs so there will be 6 blanks in the main panel. Can I use multiple 2-pole breakers (like 3 pairs) to feed the subpanel? This is because I don't know the brand of the main panel, it has 125 Main Breaker (I bought the house second hand). So I don't know the bus stab current rating so to be sure I'll feed three 2-pole feeds to the subpanel (one one bus stab won't overheat). Will this work? Has anyone seen a multiple feed like this?
Also do you see any problem if I'll just put the subpanel in front of the main panel? Any rule regarding this? I can still access the back.