Answers:
This is a two-pronged question...
Using two 7805's in parallel will consume just as much power and therefore generate just as much heat, dependent on the amount of power being drawn by the load.
The purpose of the heat sink is to allow the maximum capability of the IC without it burning itself out.
Using 2 VR's would allow for the same output as 1 without either reaching it's maximum temperature, but combined, the actual heat generated would basically be the same. Now you have to consider the ambient temperature surrounding the components and determine if that will also cause greater thermal heat on the ICs.
Basically, it really would be better to redesign the layout and insure there is adequate room for the heat sink.
It's difficult to imagine how you plan to parallel another VR and not have room for the heat sink in the first place.
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