Now answer for the OP:
What you said might be posible, but the problem is that we don't know if nintendo can tell apart a sysnand and a emunand, if nintendo can tell them apart, then what you said would not work sience nintnedo may ban you by the mere fact of having a emunand at all, but if nintendo can't tell them apart then what you said might be posible, just use 2 SD cards, and have a diferent emunand in each one, that even increase the security, you want to have the sd card for online as clean as the nand for online.