Multiple devices on a heat sink
Summary:

For typical heatsink profiles that are not used at the extreme of device failure mounting on an edge is non-ideal but will work acceptably well. This is commonly done commercially.

For heatsinks with long thin thermal paths applying heat at the geometric centre will produce significantly better results - but, the design is in any case sub-optimum.



A heatsink which symmetrical across a cross section will work best if you apply heat to its geometric center. Applying heat at an edge will be less efficient, but for a heatsink of sensible thickness the difference is not vast.
For heatsinks where the distance to thickness ratio is higher than typical the further distant points will be at lower temperature and so remove less thermal energy.
Heat flow is affected by the thermal resistance from the input point to ambient. This is made up of the conduction path through the metal sink  and then the radiation and convection path in air.
For given heat input and heat output points on the heatsink, if the combination of heatsink conductivity and material thickness  provides a much lower thermal resistance to air than the thermal resistance offered by radiation and convection then it is liable to be &quot;good enough&quot; compared to optimum.
Demonstration.
Use a compact heat source - a hot air gun or higher wattage soldering iron will do.
Mount heatsink horizontal with heat applied to center and then an edge.
If possible, adjust heat input so there is a useful gradient across the heatsink (goldilocks temperature - not too hot and not too cold).
In each case. (carefully) note the temperature at the heat input point and around the whole periphery.
Compare results.