Multiple memory mappers in MSX system
It is. I think MSX-DOS2 can manage it with no problem.
Without DOS2, using the ports, I think the ports work with the mapper you have switched on the visible slot... if you wanna manage other one, switch its slot, and then use ports as usual.
The difficult part was page 3, where system area is, and all the subslot thingy, maybe. I forgot a lot about memory mappers since I'm used to ROM 
But it was possible.
To my knowledge mapper works through ports 0FCh-0FFh, but can not figure out how to control multiple mappers through these ports (if ever possible).
Writing to ports FC~FFh selects mapper block for all mappers in the system simultanious, each mapper keeps its own copy of these registers. Reading from ports FC~FFh returns the value kept by the largest mapper in the system (as in: with the most significant bits), as far as the mappers that are present, support reading back from those regisers (not all mappers do, and programmers should never read these registers, really). You select which mapper by switching the slots they're in. It's more a software management / detection / hardware support issue.
Usually a single 1024K mapper would be more useful than 2x 512K mappers, and >4 MB? I have yet to see a useful application for that... 
Reading from ports FC~FFh returns the value kept by the largest mapper in the system (as in: with the most significant bits)
Is there any schematic example?
You select which mapper by switching the slots they're in. It's more a software management / detection / hardware support issue
Again, interesting how it can be realized in hardware.
Usually a single 1024K mapper would be more useful than 2x 512K mappers, and >4 MB? I have yet to see a useful application for that...
It was just an example... I guess megaram/megarom/scc memory mappers use other means to switch pages than fc-ff?
The memory mapper mechanism is just a 'layer' of logic on top of normal MSX slot select mechanism. And in normal speak: "memory mapper" = the RAM kind, "mapper" = any type of mapper (RAM, ROM mapper, SCC etc), depending on context.
How knows the soft if it is a mapped memory and not just plain RAM? , just testing RAM values on each switching.
I do not understand, sorry.
For all possible blocks: write block# to FC/FD/FE or FFh port, write block # in the RAM block you get, repeat. Then select all blocks again, and read back the values in RAM. A memory mapper will show different values in the memory location you wrote in (as they're different blocks you wrote to), normal RAM will just return the last value you wrote.
You have to understand memory mappers thoroughly to write a good routine for doing this (to determine memory mapper size reliably, including full 4M, non-power of 2 sizes, mappers with N blocks but 1/2N filled etc), but there's plenty examples around.
In case of two mappers used by two programs (for example, one is in "foreground" and another in interrupt background), interrupt code, after changing FC-FF values, should return them back and only then return execution to the foreground code. That's not possible without reading the port, however can use current mirror FC-FF values stored in some memory.
That's exactly what MemMan and DOS2 mapper routines are for... especially DOS2 routines are very fast, so there's no reason not to use them if available (other than laziness ) (or load MemMan before your program & use that).
