The individual blooms on both species are very look a like, I've seen C. speciosissimum with larger blooms and smaller one, briliant red or pale orange, in the wild, so the flower alone will be very similar and difficult to distinguish both species.
But the habit and leaves are is very different, C splendens, is African species with vine growth, they are climber or scrambler, similar to C thomsoniae which also an African species. Leaves are smaller around 15 -18 cm, deltoid, very short petiole and arranged in opposite, often in one plane (distichous), fruits develop later, so it is not so common to be found at the same time with blooming flower
C speciossissimum is Asian species, closely related to C paniculatum, they are shruby species, non vining. The leaves are larger and wider, with longer petiole, and arranged in opposite but on 90 degree angle (deccusate) or sometimes tricussate. Often time fruit developed quickly so it is very common to see fruits developing along the blooming flower
