I just received MegaFlashROM SCC+SD+512KB. Thanks, Manuel 
But I have an issue. When Running SD-Snatcher mapper version on previous MegaFlashROM SCC+SD, it will find SCC-I Cartridge in SLOT 1- (like it's not extended) and work with SCC music without any issues. But now when I run new version with 512KB mapper, it will restart one time and then start without issues, but cannot find any scc cartirdges. If I insert another SCC cartridge in slot 2-0, it will find it. I was testing it on Turbo-R ST & Yamaha MSX2. Both gave same result.
So the good news even with 2 mappers game will work on MegaFlashRON SCC+SD+512KB, but without music. If we compare to Playsoniq, MegaFlashROM have better results with 2 mappers in one system. If I use Playsoniq mapper on Turbo-R, game will hang. If I use Playsoniq on MSX without mapper, it will work fine. So the issue with Playsoniq is exists only with using it with 2 mappers. But Playsoniq can be used as SCC-I cartridge without any issues.
Do you press "S" when booting in DSK mode to enable SCC?
It was changed some OPFXSD versions ago due to some DSKs games writing to ROM and SCC mapper registers.
I tested with pressing "S" and indeed SCC is available again Everything works fine. I just don't understand why MSX resets on the first start, probably because of 2 mappers. The only strange thing that on MegaFlashRom SCC+SD SCC available by default even I used latest OPFXSD 1.10...
@ Guillian
Is it possible to add 1MB to the MFR SD? 
@ Colemu:
Are you watching me? (O___o!)
I have finished about 30 min ago a 1024K prototype XD
Right now I'm testing it with some big Chris animations (1400K aprox.) Seems to work fine.
Anyway, it is just a prototype. Don't expect a new model with 1024K RAM.
I have a camera in your room
I 'am' expecting a new model 
- not tempted by the 512k, but would love a 1MB one!! 