It's possible, but it won't work well.
Firstly, there is the problem of combining the two outputs, with one scaled precisely 1/256 of the other. (Whether you attenuate one by 1/256, amplify the other by 256, or some other arrangement, *16 and /16 for example, doesn't matter).
The big problem however is that an 8-bit DAC is likely to be accurate to something better than 8 bits : it may have a "DNL" specification of 1/4 LSB and an "INL" specification of 1/2LSB. These are the "Differential" and "Integral" nonlinearity specifications, and are a measure of how large each step between adjacent codes really is. (DNL provides a guarantee between any two adjacent codes, INL between any two codes across the full range of the DAC).
Ideally, each step would be precisely 1/256 of the full scale value; but a 1/4LSB DNL specification indicates that adjacent steps may differ from that ideal by 25% - this is normally acceptable behaviour in a DAC.
The trouble is that an 0.25 LSB error in your MSB DAC contributes a 64 LSB error (1/4 of the entire range) in your LSB DAC!
In other words, your 16 bit DAC has the linearity and distortion of a 10 bit DAC, which for most applications of a 16 bit DAC, is unacceptable.
Now if you can find an 8-bit DAC that guarantees 16-bit accuracy (INL and DNL better than 1/256 LSB) then go ahead : however they aren't economic to make, so the only way to get one is to start with a 16-bit DAC!
Another answer suggests "software compensation" ... mapping out the exact errors in your MSB DAC and compensating for them by adding the inverse error to the LSB DAC : something long pondered by audio engineers in the days when 16-bit DACs were expensive...
In short, it can be made to work to some extent, but if the 8-bit DAC drifts with temperature or age (it probably wasn't designed to be ultra-stable), the compensation is no longer accurate enough to be worth the complexity and expense.