Well this question is something which cannot be answered definitely as the actual answer completely relies on the authors.
First combination(Potara fused characters do a potara fusion after) : It was stated by the Old Kai in the manga that the potara earring cannot be used more than once. In other words, 2 fused characters cannot fuse with potara fusion. This is because, one fused character such as Vegito cannot take off a potara earring as both the characters would both need 2 have 1 potara earring on.Second combination(Potara fusion followed by fusion dance/Fusion dance followed by potara fusion/ Fusion dance followed by another fusion dance): Although dragon ball GT is not canon, It was indicated that even the fusion dance can only sustain a fusion powerful to a certain extent because we see SSJ4 Gogeta defuse before the time limit. We know for a fact that the potara fusion is a lot more powerful and with 2 potara fused characters (Match their power level's and fuse), the power would be enormous and they wouldn't be able to sustain the fusion. The only other option would be a bunch of weak characters first doing the potara fusion and then followed with the fusion dance. So lets say a very weak race(Humans), do the potaro fusion, for instance krillin, tien, yamcha and roshi. In the Buu saga, Base goku and base vegeta were no where as strong as Super buu Yet alone buu han. Yet we see base vegito completely owning him. So even if these 4 humans do a potara fusion followed by the fusion dance, the probability of the fusion of their fused character lasting would be very low and they would very likely immediately split back to their potara fused characters. We see goku willing to fuse with Hercule before his fight with Buuhan(Before Vegeta showed up). So goku did believe that even fusion with hercule would've made him stronger to an extent. So that leaves us with the possibility of someone as weak as Hercule and say his students doing the same. They would be able to attain the potara fusions but wouldn't have the necessary skills to perform the fusion dance. This would apply in case of all the 3 other possibilities listed in the second combination. This answer would be making an assumption such that fused characters can fuse. However, let us consider another possibility. In the Saiyan Saga, once Goku arrives to fight Vegeta and Nappa, we see him split a senzu bean and give half of it to Krillin and the other half to Gohan. We notice that this doesn't completely heal their energy. So let's say the effects of the Senzu bean were cut down to 50%? So what if Goku ate 2 successive senzu beans, instead of 1 when he's out of stamina. His body is not going to create an additional energy source after the energy from the first Senzu bean is depleted. So in other words, one could say a body is capable of only adapting to 1 fusion and another fusion wouldn't be possible?
