I agree that the income approach is the most reliable indicator. But say if the second property type is a single family residence attached to a commercial property, or a tract of excess land, and the assignment conditions call for one single property value, is it normal and proper appraisal practice to simply add the sum of the values of the distinguishable components, or apply a discount to the sum total?
Excess land is always valued separately and then added to the total; however, excess land reflects the as-is value of that area and may require a discount (EI + costs to make it a separate site ready for development) if the comparables used are already separate sites, ready for development.
Some property types are valued as a sum-of-the-whole. Vineyard/wineries, for example, may have (a) vineyards (planted and ready-to-plant land), (b) a winery building, and (c) a residence. Buyers in for that property type many times will value the individual components and sum them; sometimes at no discount.
However, for other hodgepodge properties, breaking out the components individually is probably not the way the market participants value them. Or, if they do, they would likely not apply a uniform discount. A buyer may be more interested in the warehouse space then the small house on the same site.
Every situation is unique, but in general, I'd be inclined to go with the income approach as that data (rents and expenses, and cap rates.. which might be a blend of the various uses) are probably higher quality than what you'll find in the sales comparison approach. In such a case, I might consider the sales comparison approach as a test of reasonableness only. And, I'd have to think about where to apply the adjustment in the grid for the difference in ownership: My stand-alone warehouse property likely has a higher value than my warehouse component (all other things being the same) in my hodgepodge subject if for no other reason than the comparable can be traded on a stand-alone basis; that sounds like a property rights adjustment to me.