It does not suffice to blow a very long Tekiah in order for it to count as the last Tekiah of the current set and the first Tekiah of the coming set. If one blew a long Tekiah with this intent, that it be valid for both sets, then it is proper to re-blow the entire current set starting from the first Tekiah.
First Opinion: If one lengthened the last Tekiah of a certain set (this refers to the last Tekiah of a single set of Tashrat or a single set of Tashat or a single set of Tarat) of one of the sets of Tashrat or Tashat or Tarat, the length of two Tekios in order for this Tekiah to count as two Tekiahs, the last Tekiah of the current set and the first Tekiah of the following set, nevertheless the Tekiah only counts for one set which is the last Tekiah of the current set. The reason for this is because every Tekiah must have a beginning and end and if this Tekiah were to be split to two Tekias as was the original intent of the blower then the second Tekiah would not have a beginning and the first Tekiah would not have an end. Therefore we do not follow the blowers intent and rather we consider this Tekiah as one long Tekiah and it thus counts as the last Tekiah of the current set. [590/10; See also Levush brought in Alef Hamagen 587/5]
Second Opinion: There are those that argue on the above and say that everything follows one’s intent and thus since he intended that the Tekiah count as two blows we therefore consider it as two Tekiahs and they are hence both invalid for the reason explained above. Thus it does not even count as a single Tekiah. One must thus re-blow the first Tekiah of the current set and continue with the remaining sounds just as is the law regarding one who made a mistake in middle of a set in which case he must begin from the start of that set. [See Halacha 12!]
The final ruling: The final ruling follows the first opinion. Nevertheless it is proper to suspect for the latter opinion. [590/11]
 Meaning if one is holding in the second set of Tashrat and by the last Tekiah of the second set he had intent that this Tekiah count as the last Tekiah of the second set and the first Tekiah of the third set, then he is to repeat from the first Tekiah of the second set.