TB is a bit of a corner case, essentially just for converting credit card airline credits to UA credits. Not aware of other carriers even have this option

For the normal flyer, they have mostly likely FFC (credit) and multiple of thos can easily be used together (Exception is a partial flown credit, that is the difficult case).

Far less common is ETCs and except for some very old ETCs, these are not flight credits but rather customer service issue credits. And they can for flight payments but not with FFC (but as mentioned ETCs can be converted to FFCs and then used with other FFCs.