Then, at Arcade Mode or Versus Mode

Dr. Doom

Press down once, then press down and hold it, then press Short and hold it,
Forward and hold it, and Roundhouse and hold it. Must be done quickly.
 

Thanos

Press Up once, then press up and hold it, then press Fierce and hold it, Strong
and hold it, and Jab and hold it. Must be done quickly. 
 
(M) denotes can be done in mid-air
 
Move Lists:
 
Dr. Doom
 
   Photon Beam:
               (M) B, DB, D, DF, F + P
   Photon Shock:
               (M) F, DF, D, DB, B + P
   Nebular Shield:
               F, DF, D ,DB, B + K
   Fly: (can't be canceled)
               (M) D, DB, D + 2K
 Infinity:
   Photon Array:
               (M) D,DF, F + 2P
 
Launcher: Standing Forward (2 hits)
 
Note: Photon Array can be used in an air-combo.
 
Thanos
   Dash move:
               (M) B, DB, D, DF, F + P
   Bubble Move: (temporary stun)
               D, DF, F + K
    Rock Slide:
               Hold D + Fierce
   Lava Wave:
               Hold D + Roundhouse
 
 Infinity(s):
   Reality:
              (D, DB, B) X2 + Jab
   Space:
              (D, DB, B) X2 + Strong
   Power:
              (D, DB, B) X2 + Fierce
   Time:
              (D, DB, B) X2 + Short
   Soul:
              (D, DB, B) X2 + Forward
   Mind:
              (D, DB, B) X2 + Roundhouse

Launcher: Low Strong

Note: Thanos has this short infinity combo: Activate Time Gem. Hold down and Press Roundhouse as many times as you can or until Time Gem expires.
 
Special Thanks to XYZ and Adu for making this possible.