Hey, Veyron!
The best analogy for your scenario is actually already been sticked on top of this sub-forum (Load Balancing - The Very Basic!). Basically, what you are trying to do is Link Aggregation which isn't what Multi-WAN 2.0 is meant to do. Taken from the documentation…
"As with previous multi-wan guides, this setup enables pfSense to load balance or fail over traffic from your LAN to multiple internet connections (WANs). With load balancing, traffic from the LAN is shared out on a round robin basis across the available WANs. With failover, traffic will go out the highest priority WAN until it goes down, then the next is used. pfSense monitors each WAN connection, using either the gateway IP or an IP address you provide, and if the monitor fails it will remove that WAN from use."
https://doc.pfsense.org/index.php/Multi-WAN_2.0
I will spare you the technical jargon, but every time you initiate a file download on the internet it creates a single session specific to your computer (whether it is HTTP, FTP, etc.). Multi-WAN is meant to give you load balanced based on these sessions (there is A LOT more to it than that, but I am keeping it simple). This session cannot be split by WAN links for reasons that are very technical. The only application protocol I am aware of that can aggregate multiple WAN links is BitTorrent.
Strictly speaking, I am not certain how SpeedTest is producing those results. Theoretically, Multi-WAN should assign the download test to one connection or the other (most likely the 100/10).