Is it possible to run simultaneously Eikon Desktop + Eikon Proxy API in any of these 2 scenarios?
1) With a common sub.
2) With 1 unique sub to both of the applications.
Can this be achieved with Side-by-Side API integration? What possibilities are available in order to achieve this?
You can use Eikon data retrieval APIs and Side-by-Side API in any combination: you can simultaneously run multiple applications, and each of those applications can use Eikon data retrieval APIs or Side-by-Side API, or both.