Stable 48VDC from 220VAC?
You should use a buck converter to convert 70V unstable to 48V stable. Considering the power you need, I would suggest to use a switching regulator instead of a linear one, like LM7824. You'll have better efficiency and get much less waste heat to deal with. A switching regulator with 48V output shouldn't be difficult to find.

Converting each secondary voltage to 24V is possible, but you'll have to make two converters instead of one. I would only do it if I had another 24V load to power.