Gens already has the code in it to keep the camera on Sonic in Sonic 2, it's disabled but you can enable it by uncommenting the following lines in hackdefs.h
#define SONICCAMHACK // enables camhack (see Sonic offscreen) in sonic games, and hitbox/solidity display
#define S2 // specifies that the sonic game being used is Sonic 2-based
and you might want to enable these too if you want it to look otherwise normal besides the camera position:
#define SONICNOHITBOXES // disables hitbox display of SONICCAMHACK
#define SONICNOSOLIDITYDISPLAY // disables solidity display of SONICCAMHACK
Then since you want to look at Tails instead of Sonic, there's a line in SonicHackSuite.cpp you can change:
#elif defined S2
const unsigned int P1OFFSET = 0xFFB000;
I believe Tails is at 0xFFB040 instead of 0xFFB000.
It should also be easy to change it into something non-constant if you want to be able to switch back and forth. For a really quick example that I think would work but isn't necessarily the best way to do it:
#elif defined S2
//const unsigned int P1OFFSET = 0xFFB000; // <-- disabled
// Focus on Tails if caps lock is on, or Sonic if caps lock is off
#define P1OFFSET (GetKeyState(VK_CAPITAL) ? 0xFFB040 : 0xFFB000)