The likely result of that approach is simply diluted belgian yeast character with zero lager character.
Yeast flavors come from the primary fermentation. In particular, the flavors primarily (not entirely, but almost) come from the growth phase. That is the main reason why you can raise the temperature significantly after the first couple of days (for an ale) without any significant flavor impact.
So, if you want to blend the yeast character of two different yeasts you need both yeasts to GROW during primary fermentation. Second you want them to grown in a BEER fermentation, not a starter. Ie, it should be treated exactly like a beer, oxygen before pitch, none after...temp controlled for the yeast, etc.
Starter fermentations are for growning yeast, not making good tasting beer. Beer fermentations are for making good tasting beer. These different goals are the reason for the different conditions.
To really accomplish your goal, you have two options:
1. Pitch both yeasts into your single batch of wort, at some desired ratio (50/50, 25/75, etc). Ferment as normal. Wyeast and White Labs sell a number of yeast "blends" that work exactly this way.
2. Split your wort, and pitch each with one of the yeasts. then once they are both finished beers....blend them back to taste. This approach is easier with a kegging setup, because you can carbonate both and blend the carbonated beers. If you bottle then you would have to do your blending of flat beers, which can be tricky.
Option 1, is obviously simpler because you only need one fermentation vessel. But, if you get the ratio wrong and it doesn't taste like you want...you don't have many options to "fix" it. You can brew another beer with one of the strains and try blending it back to bump up the flavor that isn't strong enough. this also requires that both yeasts work at the same fermentation temperature (blegian ale and lager yeasts don't really fit that mold).
Option 2, uses double the equipment, but gives you the ability to experiment with blending ratios glass by glass. Once you get it right, you can blend the two batches into a keg at the determined ratio (or continue to blend by the glass).
If you follow option2 and determine your desired blending ratio...then for future brews you could use option1 and pitch the yeasts at that desired ratio. it still might require some refinement because the yeasts might not grow at the same rates, resulting in some bias one way or the other. But, it should be closer than just a wild guess. Again, you would need temperature compatible yeasts.
