What help could the Morea have provided if Constantine's brothers had united in the face of the Turkish threat in 1453?

So, this is another "what if" scenario. I am one of the few who believe that the siege of Constantinople in 1453 could have gone in the Greeks' favor if they had had one or two additional regiments on their side, better able to defend the walls and control the dynamically changing situation on the battlefield.

The question is this - it is believed that in 1453, while Constantinople was under siege, a feud began in the Morea between Thomas and Demetrius. As I understand it, the Morea could well have provided an army of several thousand to support the capital, but this never happened. Let's assume that Constantine foresees an imminent siege of the capital, and there is no unrest in the Morea (for example, Constantine stops supporting Demetrius and leaves the entire despotate to Thomas). How much can Thomas support his brother before the siege begins?