If your sample rate is that low, then you'll have to seriously low-pass filter the current measurements themselves; in which case, "simultaneous" sampling is useless IMHO. You can just use one ADC and an additional analog multiplexer.
Note that that's what the ADS1158 does - it just has an internal analog MUX. You'll need two of them. They are about $10 each (for low quantities.) I don't see what the problem is with its SPI interface.
If the voltage span fits your requirements, you could consider using a 32-channel MUX instead, such as the ADG731, and then a single-input 16-bit ADC (if you really need 16-bit?) That's still two chips, but that'll be cheaper. Oh and if 12-bit is enough, you could consider using the internal ADC of some MCU, many do have 12-bit ADCs these days. The ADG731 itself is about $5. It's available in TQFP, which is easy to hand solder.