You could simply HFSC shape outbound WAN (uploads) with a 100Mbit upperlimit. If you make a queue for each LAN subnet and give them both the same queue settings it should evenly share the pipe.
Downloads are trickier since you have two LANs. I see in the other thread you're using CARP so I presume you have a pfsync setup so that eliminates the use of limiters due to a bug.
The only way I know of to do this with multi-LAN and HFSC is to place a node between you and the ISP shaping the traffic out one interface and into another node with LAN and OPT1.
Gig-E –- Shaper node --- Gig-E --- WAN --- existing node --- LAN & OPT1
Or you could use a 50Mbit upperlimit on each interface and they would not be able to borrow from each other.
All in all you cannot control how fast traffic arrives from your ISP. Can they rate-limit what they send to you?