The "X-blade" shown in BBS was only 2/20 of the real deal it seems.
The X-blade is created by a high dimensional clash between two poles consisting of the powers of light and darkness, as stated by the KH DDD Glossary, it doesn't talk specifically about hearts:
DDD Glossary said:
The letter χ can be pronounced both "key" and "kye" (leading to some confusion) and symbolizes the perfect crossing of light and darkness. The χ-blade can be forged via a high-dimensional clash between those two poles.
While Ventus' resistance against the fusion process did serve to make the weapon even more frail, the BBS-version has nowhere the power the "perfect" one is supposed to have.
Xehanort said he didn't do it the "right" way because he was conducting another experiment and trying to use a shortcut in order to obtain the X-blade.
He hoped/theorized that by using
one heart of
pure light and
one of
pure darkness would make up for the lesser amount of pieces involved in it and thus he wouldn't have to search for actual twenty pieces.
Xehanort also admitted that he acted rashly/hasty, but he had nonetheless the backup plan with the 7/13 pieces already prepared.
Since Xehanort cares for no one but himself, regardless how the result of the experiment in BBS was, he himself would not really lose something of importance. If it worked, Xehanort would have won right then and used that X-blade to start another Keyblade War. If it didn't work (which is the true outcome), he would just move to the next contingency plan he made.
