Not just another tm1-ui-chart-value formula, but another set/series of data.
For example, if you have values in the multi bar chart as series A:
100, 200, 500
Then, you should have another series that is let us say, series B, that has:
200, 300, 100
The multi bar, will "pair" 100 and 200 from series A and B, and will do so for the rest. These are what the multi bar chart is stacking up (100 over 200, 200 over 300, 500 over 100).
Let us know how it goes.