It would be good to have an official Mikrotik answer, but I don’t think the lines are so clear divided. They look to me something more like the dual channel memory we have in computers: both channels are just used by what is needed.
Take a look at this pdf. The block diagram is at page 2
http://www.t-firefly.com/download/FireWRT/hardware/MT7621.pdf
When you want an official MikroTik answer, why not get the info from the MikroTik website instead of from other sites?
https://mikrotik.com/product/RB750Gr3#fndtn-downloads
(I think) all MikroTik equipment uses hidden VLANs to make the indivudual ports accessible on an architecture like that.
This is also the reason why VLAN switching is not accelerated on that device: the VLAN layer is already used by the device itself, user defined VLANs are handled in software above that.
Of course when you decide to run different software on it, you can also decide to do it in a completely different way.