Why not use an external ADC? (SPI/I2C interface)
There's plenty to choose from and that way you can use a favorite/cheap microcontroller. For two channels you can get them in 8-pin packages - no big deal for space and I'm sure it will be cheaper overall.
Why not, indeed, and separate ADC is probably the way to go if you can't find a suitable one with integrated ADCs. Separate may also give better performance.
But, it appears that today, cheap microcontrollers -- or those I could consider calling "favorite" --, tend to have dual or triple ADCs -- unless the "favorite" means some 1990's 8-bit controller families, which, by the way, aren't any cheaper than the modern ones, some mid-range AVRs are ridiculously expensive for what they have.
So, in most cases, if you just can't find a suitable MCU with dual ADCs and that's the only reason you consider going to a separate ADC, you are not looking very well at modern microcontrollers. Most modern $2-$3 MCUs do dual ADC along with a ton of other very useful peripherals that greatly simplifies CPU tasks compared to an AVR or PIC, such as DMA, which is a huge blessing from the software simplicity viewpoint, especially with multi-channel, multi-ADC.